How do I call a function on my level-blueprint from my gamestate-blueprint?

How do I call a function on my level-blueprint from my gamestate-blueprint?

I want to start a matinee as soon as my game transitions into the intro-state (have setup a little custom state-machine that handles transitions into additional states)

Actors have no access to a Level Blueprint.
To fire an event/function from the level blueprint, you should use event dispatchers:
https://docs.unrealengine.com/latest/INT/Engine/Blueprints/UserGuide/EventDispatcher/Calling/index.html#callinginlevelblueprints