Gameplay Ability System

Links and sources

Example of using the GAS: GASShooter GitHub project

Unofficial but detailed GAS Documentation

Epic livestream in depth look at GAS

Setting up GAS youtube tutorial

Notes

Any Actor that wishes to use GameplayAbilities, have Attributes, or receive GameplayEffects must have one AbilitySystemComponent (ASC) attached to them.
ASC can be assigned to weapons, players, or AI

The Actor with the ASC attached to it is referred to as the OwnerActor of the ASC.
The physical representation Actor of the ASC is called the AvatarActor
The OwnerActor and AvatarActor can either be the same or different depending on the use case

If your Actor will respawn and need persistence of Attributes or GameplayEffects between spawns (like a hero in a MOBA), then the ideal location for the ASC is on the PlayerState.

Setup

In the sections below, my game is named unrealgame5, and any appearances of this string should be replaced by your own project name.

Project Plugins and Modules

Initial contents of unrealgame5.build.cs

using UnrealBuildTool;

public class unrealgame5 : ModuleRules
{
	public unrealgame5(ReadOnlyTargetRules Target) : base(Target)
	{
		PCHUsage = PCHUsageMode.UseExplicitOrSharedPCHs;

		PublicDependencyModuleNames.AddRange(new string[] {"Core", "CoreUObject", "Engine", "InputCore", "HeadMountedDisplay"});
  }
}

Add "GameplayAbilities", "GameplayTags", "GameplayTasks" modules to unrealgame5.build.cs

using UnrealBuildTool;

public class unrealgame5 : ModuleRules
{
	public unrealgame5(ReadOnlyTargetRules Target) : base(Target)
	{
		PCHUsage = PCHUsageMode.UseExplicitOrSharedPCHs;

		PublicDependencyModuleNames.AddRange(new string[] { "Core", "CoreUObject", "Engine", "InputCore", "HeadMountedDisplay" });
		PublicDependencyModuleNames.AddRange(new string[] { "GameplayAbilities", "GameplayTags", "GameplayTasks" });
	}
}

Then within the editor go to Edit->Plugins... and enable the Gameplay Abilities plugin.

C++ Initial Setup

First, we need to modify the contents of the header file for our unreal project. My project is named unrealgame5 so the file is unrealgame5.h, and the contents are below. If you already have information here, just make the the EGASAbilityInputID enumeration is added to the header file and save your changes.

// Copyright Epic Games, Inc. All Rights Reserved.

#pragma once

#include "CoreMinimal.h"

UENUM(BlueprintType)
enum class EGASAbilityInputID : uint8
{
  None, 
  Confirm, 
  Cancel,
  Attack
};

To finish the setup you will at least need to define the following new source files for your project.

For my project I follow the same naming convention used with GASDocumentation. I'll paste it below, but for the first few files required to set up the Gameplay Ability System (GAS), we are defining the backend of the system so none of these conventions apply. For these files, I added the GAS_ convention.

Prefix 	Asset Type
GA_ 	GameplayAbility
GC_ 	GameplayCue
GE_ 	GameplayEffect
GAS_    GameplayAbilitySystem (Core Configurations)

Next, we need to create an AbilitySystemComponent. To do this, open your project in unreal and create a new C++ source file, inheiriting from the AbilitySystemComponent base class.

I named this class GAS_AbilitySystemComponent, and the generated files are below

// GAS_AbilitySystemComponent.h
// All content (c) Shaun Reed 2021, all rights reserved

#pragma once

#include "CoreMinimal.h"
#include "AbilitySystemComponent.h"
#include "GAS_AbilitySystemComponent.generated.h"

/**
 * 
 */
UCLASS()
class UNREALGAME5_API UGAS_AbilitySystemComponent : public UAbilitySystemComponent
{
	GENERATED_BODY()
	
};

// GAS_AbilitySystemComponent.cpp
// All content (c) Shaun Reed 2021, all rights reserved

#include "GAS_AbilitySystemComponent.h"

Next, we need to create an AttributeSet for our game. Repeat the process of creating a new C++ sourcce file for your ue5 project, but this time inherit from AttributeSet

I named this class GAS_AttributeSet and the files genereated are below

// GAS_AttributeSet.h
// All content (c) Shaun Reed 2021, all rights reserved

#pragma once

#include "AbilitySystemComponent.h"

#include "CoreMinimal.h"
#include "AttributeSet.h"
#include "GAS_AttributeSet.generated.h"

// Macros to define getters and setters for attributes (AttributeSet.h)
#define ATTRIBUTE_ACCESSORS(ClassName, PropertyName) \
		GAMEPLAYATTRIBUTE_PROPERTY_GETTER(ClassName, PropertyName) \
		GAMEPLAYATTRIBUTE_VALUE_GETTER(PropertyName) \
		GAMEPLAYATTRIBUTE_VALUE_SETTER(PropertyName) \
		GAMEPLAYATTRIBUTE_VALUE_INITTER(PropertyName)

/**
 *
 */
UCLASS()
class UNREALGAME5_API UGAS_AttributeSet : public UAttributeSet
{
	GENERATED_BODY()
    
  UGAS_AttributeSet();

public:
	virtual void GetLifetimeReplicatedProps(TArray<FLifetimeProperty>& OutLifetimeProps) const override;

	/*
	* Attribute Definitions
	*/

	// Health

	UPROPERTY(BlueprintReadOnly, Category = "Attributes", ReplicatedUsing = OnRep_Health)
	FGameplayAttributeData Health;
	// Use macros we defined from AttributeSet.h to generate getters and setters
	ATTRIBUTE_ACCESSORS(UGAS_AttributeSet, Health);

	UFUNCTION()
		virtual void OnRep_Health(const FGameplayAttributeData& OldHealth);

	// Stamina

	UPROPERTY(BlueprintReadOnly, Category = "Attributes", ReplicatedUsing = OnRep_Stamina)
	FGameplayAttributeData Stamina;
	ATTRIBUTE_ACCESSORS(UGAS_AttributeSet, Stamina);

	UFUNCTION()
		virtual void OnRep_Stamina(const FGameplayAttributeData& OldStamina);

	// Attack Power

	UPROPERTY(BlueprintReadOnly, Category = "Attributes", ReplicatedUsing = OnRep_AttackPower)
	FGameplayAttributeData AttackPower;
	ATTRIBUTE_ACCESSORS(UGAS_AttributeSet, AttackPower);

	UFUNCTION()
		virtual void OnRep_AttackPower(const FGameplayAttributeData& OldAttackPower);
};

// GAS_AttributeSet.cpp
// All content (c) Shaun Reed 2021, all rights reserved

#include "Net/UnrealNetwork.h"  // DOREPLIFETIME
#include "GAS_AttributeSet.h"

UGAS_AttributeSet::UGAS_AttributeSet()
{
}

void UGAS_AttributeSet::GetLifetimeReplicatedProps(TArray<FLifetimeProperty>& OutLifetimeProps) const
{
  Super::GetLifetimeReplicatedProps(OutLifetimeProps);
  
  DOREPLIFETIME_CONDITION_NOTIFY(UGAS_AttributeSet, Health, COND_None, REPNOTIFY_Always);
  DOREPLIFETIME_CONDITION_NOTIFY(UGAS_AttributeSet, Stamina, COND_None, REPNOTIFY_Always);
  DOREPLIFETIME_CONDITION_NOTIFY(UGAS_AttributeSet, AttackPower, COND_None, REPNOTIFY_Always);

}

void UGAS_AttributeSet::OnRep_Health(const FGameplayAttributeData& OldHealth)
{
  GAMEPLAYATTRIBUTE_REPNOTIFY(UGAS_AttributeSet, Health, OldHealth);
}

void UGAS_AttributeSet::OnRep_Stamina(const FGameplayAttributeData& OldStamina)
{
  GAMEPLAYATTRIBUTE_REPNOTIFY(UGAS_AttributeSet, Stamina, OldStamina);
}

void UGAS_AttributeSet::OnRep_AttackPower(const FGameplayAttributeData& OldAttackPower)
{
  GAMEPLAYATTRIBUTE_REPNOTIFY(UGAS_AttributeSet, AttackPower, OldAttackPower);
}

C++ Character Setup

In the files below, my character is named ThirdPersonCharacter, so any appearances of this string may need to be replaced with your character's name instead. To setup your character that inherits from ACharacter base class, make the following changes to your files.

In the ThirdPersonCharacter.h file, make sure you're inheriting from public IABilitySystemInterface. The start of your class should look like this. Pay attention to the includes.

// All content (c) Shaun Reed 2021, all rights reserved

#pragma once

// GAS includes
#include "AbilitySystemInterface.h"
#include <GameplatEffectTypes.h>

#include "CoreMinimal.h"
#include "GameFramework/Character.h"
#include "ThirdPersonCharacter.generated.h"

UCLASS()
class UNREALGAME5_API AThirdPersonCharacter : public ACharacter, public IAbilitySystemInterface
{
	GENERATED_BODY()

public:

// more code....

Next, we add an instance of our GAS_AbilitySystem class using the UGAS_AbilitySystemComponent typename, and we also add an instance of our GAS_AttributeSet class using the UGAS_AttributeSet type.

UCLASS()
class UNREALGAME5_API AThirdPersonCharacter : public ACharacter, public IAbilitySystemInterface
{
	GENERATED_BODY()

public:
	// GAS declarations
	UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = "GAS")
	class UGAS_AbilitySystemComponent* AbilitySystemComponent;

	UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = "GAS")
    class UGAS_AttributeSet* Attributes;
// more code....

Next, we declare the required virtual functions that we will need to define to use the GAS.

UCLASS()
class UNREALGAME5_API AThirdPersonCharacter : public ACharacter, public IAbilitySystemInterface
{
	GENERATED_BODY()

public:
	// GAS declarations
	UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = "GAS")
	class UGAS_AbilitySystemComponent* AbilitySystemComponent;

	UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = "GAS")
    class UGAS_AttributeSet* Attributes;
    
	virtual class UAbilitySystemComponent* GetAbilitySystemComponent() const override;
    
// more code....

We need to implement GetAbilitySystemComponent, which simply returns our UGAS_AbilitySystemComponent component.

// ThirdPersonCharacter.cpp
UAbilitySystemComponent* AThirdPersonCharacter::GetAbilitySystemComponent() const
{
	return AbilitySystemComponent;
}

Next, we need to modify the character's constructor to add the components we decalred. I removed the code from my constructor that wasn't related to the GAS. The additions are below.

// ThirdPersonCharacter.cpp

AThirdPersonCharacter::AThirdPersonCharacter()
{
 	// Initializing any components unrelated to GAS...
	// ...

	
}

Next we need to overload a virtual function InitializeAttributes() to handle initializing the attributes for our game at the start. We also declare the DefaultAttributeEffect member variable to help define and apply the default attributes for our character.

// ThirdPersonCharacter.h
UCLASS()
class UNREALGAME5_API AThirdPersonCharacter : public ACharacter, public IAbilitySystemInterface
{
	GENERATED_BODY()

public:
	// GAS declarations
	UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = "GAS")
	class UGAS_AbilitySystemComponent* AbilitySystemComponent;

	UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = "GAS")
    class UGAS_AttributeSet* Attributes;
    
	virtual class UAbilitySystemComponent* GetAbilitySystemComponent() const override;
    
    // Add these lines
  	virtual void InitializeAttributes();
    UPROPERTY(BlueprintReadOnly, EditDefaultOnly, Category = "GAS")
    TSubclassOf<class UGameplayEffect> DefaultAttributeEffect;
// more code....

Define InitializeAttributes()

// ThirdPersonCharacter.cpp
void AThirdPersonCharacter::InitializeAttributes()
{
	// If the ASC and DefaultAttributeEffect objects are valid
	if (AbilitySystemComponent && DefaultAttributeEffect)
	{
		// Create context object for this gameplay effecct
		FGameplayEffectContextHandle EffectContext = AbilitySystemComponent->MakeEffectContext();
		EffectContext.AddSourceObject(this);

		// Create an outgoing effect spec using the effect to apply and the context
		FGameplayEffectSpecHandle SpecHandle = AbilitySystemComponent->MakeOutgoingSpec(DefaultAttributeEffect, 1, EffectContext);

		if (SpecHandle.IsValid())
		{
			// Apply the effect using the derived spec
			// + Could be ApplyGameplayEffectToTarget() instead if we were shooting a target
			FActiveGameplayEffectHandle GEHandle = AbilitySystemComponent->ApplyGameplayEffectSpecToSelf(*SpecHandle.Data.Get());
		}
	}
}

Similar to how we defined default attributes, we define default abilities for our character by overloading the GiveAbilities() function. We also add the DefaultAbilities array to store the default abilities for the character.

// ThirdPersonCharacter.h
UCLASS()
class UNREALGAME5_API AThirdPersonCharacter : public ACharacter, public IAbilitySystemInterface
{
	GENERATED_BODY()

public:
	// GAS declarations

	// Define components to store ASC and attributes
	UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = "GAS")
	class UGAS_AbilitySystemComponent* AbilitySystemComponent;
	UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = "GAS")
  class UGAS_AttributeSet* Attributes;

	virtual class UAbilitySystemComponent* GetAbilitySystemComponent() const override;
	// Overload to initialize attributes for GAS, and component to store default attributes
	virtual void InitializeAttributes();
	UPROPERTY(BlueprintReadOnly, EditDefaultOnly, Category = "GAS")
  TSubclassOf<class UGameplayEffect> DefaultAttributeEffect;

	// Overload to initialize abilities for GAS, and component to store default abilities
	virtual void GiveAbilities();
	UPROPERTY(BlueprintReadOnly, EditDefaultOnly, Category = "GAS")
	TArray<TSubclassOf<class UGAS_GameplayAbility>> DefaultAbilities;
public:	
// more code....

And we define GiveAbilities below

// ThirdPersonCharacter.cpp
void AThirdPersonCharacter::GiveAbilities()
{
	// If the server has the authority to grant abilities and there is a valid ASC
	if (HasAuthority() && AbilitySystemComponent)
	{
		// Foreach ability in DefaultAbilities, grant the ability
		for (TSubclassOf<UGAS_GameplayAbility>& StartupAbility : DefaultAbilities)
		{
			// `1` below is the level of the ability, which could later be used to allow abilities to scale with player level
			AbilitySystemComponent->GiveAbility(
				FGameplayAbilitySpec(StartupAbility, 1, static_cast<int32>(StartupAbility.GetDefaultObject()->AbilityInputID), this));
		}
	}
}

For the next step, we need to override PossessedBy and OnRep_PlayerState() to define how to update the server and client of the player state respectively.

// ThirdPersonCharacter.h
UCLASS()
class UNREALGAME5_API AThirdPersonCharacter : public ACharacter, public IAbilitySystemInterface
{
	GENERATED_BODY()

public:
	// GAS declarations

	// Define components to store ASC and attributes
	UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = "GAS")
	class UGAS_AbilitySystemComponent* AbilitySystemComponent;
	UPROPERTY(VisibleAnywhere, BlueprintReadOnly, Category = "GAS")
  class UGAS_AttributeSet* Attributes;

	virtual class UAbilitySystemComponent* GetAbilitySystemComponent() const override;
	// Overload to initialize attributes for GAS, and component to store default attributes
	virtual void InitializeAttributes();
	UPROPERTY(BlueprintReadOnly, EditDefaultOnly, Category = "GAS")
  TSubclassOf<class UGameplayEffect> DefaultAttributeEffect;

	// Overload to initialize abilities for GAS, and component to store default abilities
	virtual void GiveAbilities();
	UPROPERTY(BlueprintReadOnly, EditDefaultOnly, Category = "GAS")
	TArray<TSubclassOf<class UGAS_GameplayAbility>> DefaultAbilities;
public:	
// more code....

And see the definitions for these functions below

// ThirdPersonCharacter.cpp
void AThirdPersonCharacter::PossessedBy(AController* NewController)
{
	Super::PossessedBy(NewController);

	// Owner and Avatar are bother this character
	AbilitySystemComponent->InitAbilityActorInfo(this, this);

	InitializeAttributes();
	GiveAbilities();
}

void AThirdPersonCharacter::OnRep_PlayerState()
{
	Super::OnRep_PlayerState();

	AbilitySystemComponent->InitAbilityActorInfo(this, this);
	InitializeAttributes();

	if (AbilitySystemComponent && InputComponent)
	{
		// Where the 3rd parameter is a string equal to enum typename defined in unrealgame5.h
		const FGameplayAbilityInputBinds Binds("Confirm", "Cancel", "EGASAbilityInputID", static_cast<int32>(EGASAbilityInputID::Confirm), static_cast<int32>(EGASAbilityInputID::Cancel));
		AbilitySystemComponent->BindAbilityActivationToInputComponent(InputComponent, Binds);
	}
}

As a final modification, add the following code to the function equivalent to ThirdPersonCharacter::SetupPlayerInputComponent() in your project. This is the same code as the last portion of OnRep_PlayerState

void AThirdPersonCharacter::SetupPlayerInputComponent(UInputComponent* PlayerInputComponent)
{
	// Code unrelated to GAS...
	// ...  
  
	// Make sure GAS is valid along with player input component
	if (AbilitySystemComponent && InputComponent)
	{
		// Where the 3rd parameter is a string equal to enum typename defined in unrealgame5.h
		const FGameplayAbilityInputBinds Binds("Confirm", "Cancel", "EGASAbilityInputID", static_cast<int32>(EGASAbilityInputID::Confirm), static_cast<int32>(EGASAbilityInputID::Cancel));
		AbilitySystemComponent->BindAbilityActivationToInputComponent(InputComponent, Binds);
	}
}

And just to be sure we have all the headers we need, here are the final includes for my character

// ThirdPersonCharacter.h
// GAS includes
#include "AbilitySystemInterface.h"
#include <GameplayEffectTypes.h>
#include "GAS_AbilitySystemComponent.h"
#include "GAS_GameplayAbility.h"
#include "GAS_AttributeSet.h"

#include "CoreMinimal.h"
#include "GameFramework/Character.h"
#include "ThirdPersonCharacter.generated.h"

// ThirdPersonCharacter.cpp
// All content (c) Shaun Reed 2021, all rights reserved

#include "ThirdPersonCharacter.h"

// Custom includes (Not related to GAS)
#include "ActorSpawner.h"  // Fireball spawner object
#include "BallActor.h"  // Fireball object

// Includes for GAS
#include "../unrealgame5.h"

// Engine includes
#include "Kismet/GameplayStatics.h" // For spawning fireball static mesh
#include "Camera/CameraComponent.h"
#include "GameFramework/SpringArmComponent.h"
#include "GameFramework/CharacterMovementComponent.h"

Defining Abilities

At this point we have configured GAS for our project and our character, so we're ready to start defining our abilities!