GC FSM is a plugin for Unreal® Engine that allows you to create event-driven, hierarchical finite state machines directly in blueprints.

Why do you need FSMs when you have behaviour trees, you ask? BTs are great, but they require quite a bit of machinery. For instance, BTs must be run by an AIController and also need a Pawn actor. But there are tasks that have complex logic that are not "behaviours" in the strictest sense and that are not logically run by an "enemy". Think about menus, puzzles or even parts of the logic of your PlayerController. Can you use BTs for those? Probably yes, but not without making it more complicated than it should be. GC FSM allows you to implement those tasks using FSMs right in blueprints, for all kind of objects: actors, components, widgets, etc. with ease and without any gimmick to use a tool clearly designed for something else.