What do you do if the guy you love has a gf?
You back off.
Stare from a distance. Because that is all you can and should do.
Back off..don't think about interfering with their relationship.
Personally?... I'd relize he is straight and find someone else.... For you?... There are more minnows in the pond...
Don't even get involved you'll find better.
But what if the gf is cheating
Move on. :)
Be friends with him... So when she dumps him, you make your move :) jk..
You'll get over him, trust me. And ask yourself why you even like him and try to think of a good answer for that question.