Something as simple as GWorld->GetLevel(0) will retrieve the global level of the current GWorld.
However, it’s not entirely advisable to use GWorld (we try and limit the use of it internally for more robust code that can utilize multiple UWorld’s).
Passing in an Actor and using GetLevel() on it will retrieve the ULevel it is a part of. You would require to have an Actor capable of being sent when you call the function, however.