Gameplay Ability System (GAS)
Unreal Engine's framework for abilities, attributes and effects — built for Paragon, then Fortnite, with network prediction baked in.
A fireball, a shield, a sprint and a poison tick look nothing alike but are made of the same parts. GAS pulls them apart: a GameplayAbility is the action, a GameplayEffect modifies values in an AttributeSet (health, mana, move speed), and a GameplayCue handles sound and particles. States like "stunned" or "on cooldown" are expressed as gameplay tags rather than flags in code.
The real payoff is multiplayer. Abilities can fire instantly on the player's machine using client-side prediction and roll back if the server disagrees. The cost is a steep learning curve and thin official docs; for years a community-written GitHub guide was the manual most people actually read.