I think that dogs can change their affections, so I wouldn't worry about it too much if she is going to be spending more time with the dog than you at some point.
Britster gives good advice as usual (
). If possible have your gf feed him. Also, FYI, I became my dog's favorite person while he belonged to someone else, simply by being more fun. I took him on walks, she did not, I trained him, she did not. So maybe your dog can have some special fun time with your gf that he does not get otherwise -- special treats might help with this too!
I'm sure he will love both of you, and it's only been a few days. Try to be happy that the dog is happy.
