Girlfriends dad does not like me. What should I do?

My guess is the age. You're legal, she is not. If you are a decent person, then you have nothing to worry about. Be polite to him, and earn his respect, and he will then begin to trust her more, and let her go out with you more. Try to figure out what he likes...find what he's interested in and see what your common areas are. Try to relate to him, but don't be fake about it.