Gameplay Ability System (GAS)¶

GAS is UE's typed gameplay logic framework. It models abilities, attributes, gameplay effects, and tags in a replicated, predictable, designer-friendly way. Used by Lyra and most modern AAA gameplay built on UE 5.

Enable¶

Project Settings → Plugins → Gameplay Abilities → restart.

Core types¶

Class	Purpose
`UAbilitySystemComponent` (ASC)	Attached to a Pawn/Character; the GAS hub
`UGameplayAbility`	An ability — granted to ASCs, can be activated
`UGameplayEffect`	A modifier — applies to attributes (damage, buffs)
`UAttributeSet`	A typed set of replicated attributes
`FGameplayTag`	Hierarchical tags (e.g. `State.Status.Stunned`)
`UGameplayCue`	Cosmetic effects fired by tag (sounds, VFX)

Activation flow¶

sequenceDiagram
    Player->>ASC: InputAction "FireGun"
    ASC->>Ability: TryActivateAbility(FireGunAbility)
    Ability->>Ability: CommitAbility() — pay cost, check cooldown
    Ability->>ASC: ApplyGameplayEffect(DamageEffect to Target.ASC)
    Target.ASC->>AttributeSet: Health -= Damage
    AttributeSet->>UI: OnAttributeChanged broadcast

Attributes¶

UAttributeSet declares replicated floats:

UCLASS()
class UHeroAttributeSet : public UAttributeSet
{
    GENERATED_BODY()
public:
    ATTRIBUTE_ACCESSORS(UHeroAttributeSet, Health);
    UPROPERTY(BlueprintReadOnly, ReplicatedUsing=OnRep_Health)
    FGameplayAttributeData Health;
    UFUNCTION() void OnRep_Health(const FGameplayAttributeData& Old);
};

Gameplay Effects¶

Three durations:

Instant — single change (heal +20)
Duration — temporary buff (10 s strength boost)
Infinite — until removed by tag/effect

Effects can apply modifiers, grant abilities, or fire gameplay cues.

Tags¶

Hierarchical strings: Ability.Combat.Slash matches a tag query for Ability.Combat.*. Tag tables drive ability requirements, cancel/block rules, and gameplay cue routing.

Prediction¶

GAS supports client-side prediction with server reconciliation — the client guesses the ability's outcome, the server validates, mismatches trigger rollback. For each ability, set:

NetExecutionPolicy = EGameplayAbilityNetExecutionPolicy::LocalPredicted;

When to use vs hand-rolled gameplay¶

Need	GAS	Hand-rolled
Attribute mutation w/ replication	★	Lots of boilerplate
Effect stacking / cancellation rules	★	Custom queue
Designer-tunable effects	★	Hardcoded
Tag-driven AI state	★	Manual flags
Very simple games	Overkill	★

Gotchas¶

The ASC must be replicated for multiplayer GAS. Default Lyra setup handles this.
AttributeSet initialization order matters — set defaults in the ASC's BeginPlay or via a startup UGameplayEffect.
Abilities authored as BP Blueprints have slower activation than C++ abilities; keep critical path abilities in C++.