How to do it...

1. If you have not done so already, complete Steps 1-4 of the GameplayAbilities API – triggering an actor's gameplay abilities with game controls recipe to link to the GameplayAbilities API in your ProjectName.Build.cs file and enable its functionality.
2. Create a new C++ class  by going to the Content Browser and selecting Add New | Add C++ Class. From the Add C++ Class menu, check the Show All Classes option. From there, type in attr and select AttributeSet as your parent class. From there, click the Next button:

3. Give the class a Name of GameUnitAttributeSet and click on Create Class:

Once created, deck the class out with a set of UPROPERTY specifiers that you want each Actor to have in their property set.

4. For example, you might want to declare your UAttributeSet derivate class similar to what's given in the following piece of code:

#pragma once

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

/**
 * 
 */
UCLASS(Blueprintable, BlueprintType)
class CHAPTER_11_API UGameUnitAttributeSet : public UAttributeSet
{
    GENERATED_BODY()
    
public:
    UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = GameUnitAttributes)
    float Hp;

    UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = GameUnitAttributes)
    float Mana;

    UPROPERTY(EditAnywhere, BlueprintReadWrite, Category = GameUnitAttributes)
    float Speed;
};

If your code is networked, you might want to enable replication on each of the UPROPERTY specifiers with the replicated declaration in the UPROPERTY macro.

5. Connect GameUnitAttributeSet with your AbilitySystemComponent inside your Actor class. We can do this with the Warrior class we created previously by opening the Warrior.h file and adding the following function declaration:

virtual void PostInitializeComponents() override;

6. Then, open Warrior.cpp and add the following #include:

#include "GameUnitAttributeSet.h"

7. Afterwards, implement that function:

void AWarrior::PostInitializeComponents()
{
    Super::PostInitializeComponents();

    if(AbilitySystemComponent)
    {
        AbilitySystemComponent->InitStats(UGameUnitAttributeSet::StaticClass(), NULL);
    }
}

You can put this call somewhere in PostInitializeComponents(), or in code that is called later than that.

8. Once you have registered UAttributeSet, you can move on with the next recipe and apply GameplayEffect to some of the elements in the attribute set.

9. Be sure your Actor class object implements IAbilitySystemInterface by deriving from it. This is extremely important as the UAbilitySet object will attempt a cast to IAbilitySystemInterface to call GetAbilitySystemComponent() on it at various places in the code.