I never say GG, or GF, I always say ''Dammit'' or''Dammitall'' really, ask Suomynona!
Saying good game or good fight is a sign of respect towards your fellow toribasher. So I think it should be said no matter if the match was really legitimate or not.... Just my humble opinion.
I do one of two things. I gloat and rub it in their face for the next five hours if I win, or if I lose, I throw a tantrum and then proceed to go on a five hour rant about how he simply won due to the fact that I was distracted, and I was distracted, because of my upbringing and blah blah.