Commonly confused with 'g2g' which means "got to go". They are different, and 'gtg' is normally used in online games to indicate a participant's "ready status." It is seeing common usage today as indicating in the example.
most people know that gtg means "got to go" or "good to go", but not many people know that gtg can also mean "gay to gay". You use this last definition when someone is engaging in gay sexual behavior.