mixed meaning, in the case of a friend, good for you. If someone you don't like, Go F*** Yourself
Friend: Congrats on the promotion, G.F.Y.
Jerk: Leave me alone, G.F.Y.
Go Fuck Yourself,Good For You, God Forgives You. G.F.Y. always means Go Fuck Yourself but the other two options can be used if you're scared to tell the truth or if you're talking to your mother
Hey Mike give me a call later and in the mean time G.F.Y.