I think I am gay....?

Firstly, I get how difficult it can be to figure out your uality. I'm glad you are okay with being or gay. Second, you should handle this similarly to how you would handle this situation with a boy. Be friends with her, but don't be the girl she cheats with. That will not be healthy for either you or her, and it will break her girlfriend's heart. Try to have a good friendship with her and if she breaks up with her girlfriend it could develop into something more. You probably shouldn't sleep in the same bed with her either, it could lead to her cheating with you.