This debate has occurred on many forums in the past. There are a few schools of thought on the topic of GG etiquette. I'd like to hear some thoughts on it from you guys though. I was prompted to talk about it when I heard Chill and djWHEAT talk about it on djWHEAT's Weapon of Choice show (which is awesome btw).
Personally, I think if you get offended when someone leaves without saying GG, you've got a bit of a stick up your arse. If you think GG is a nice thing to do after a game, that's an expectation you've developed yourself over time. Nobody owes it to you to say GG. Sure, GG might be a polite thing to do, but that does not mean that omitting GG at the end of your play is an impolite thing to do.
Personally, 99% of the time I actually do say GG at the end of the game because it's my way of saying "I surrender, well played", but I would never criticise someone for not doing the same, because I'm not a fan of volunteering other people into my own etiquette on their behalf. I don't have that right.
Do you get offended if you beat someone and they don't GG?