On a blitz game I think 'gg' is appropriate.. However in a game with multiple days per move I will actually take time out of my busy schedule to spell 'good game.'
gg is arrogance

ok im just sayin here, dont mind me at all, but gg means owned. if your gona play me and not even say hello to me or anythin, and you say "gg" after you win, your basicly sayin 'hahahaha noob owned lol'. im just letin y'all know. if its a friends game or whatever obviously say whatever you want but if not, dont say gg to me. im just sayin. kthxbai :)
gg is supposed 2 mean good game not you got owned

but if you really had a large material/positional advantage and they resigned i probably wouldn't say gg
it's only a good game if the loss was so close that a discovery seals the victory!
then ... gg is appropriate.
Everything reasonable that could be say regarding this topic probably has been said alredy.
Personally I say very little in a match, and almost always it is a reply to the other guy saying hi. If I won the game I will say nothing unless the opponent says something first. If the other guy said hello in the start, and I lost the game, I may say have a nice day.
The problem I would have with someone saying gg is that people really only say it after they have won, 19 out of 20 times, it has been said by the opponent is after I have lost.
It's the same with rematch, when you are tired, losing badly people will ask for a rematch so that they can beat you again, when you win people will not ask for a rematch.
A lot people likes to say gg after they have got what would have been considerd a cheap win. Games where you have a big advantage for a long time, they have been outplayed t. You have had the option of exchanging off pieces, and then winning easily. instead you play on, make a blunder of some sort.,the opponent begin exchanging off all the remanining pices, wins and says gg.
I would prefer people saying either nothing, or something like, have a nice day, take care, you are a good player -instead of simply gg. Any of these comments I would like more then gg. if you are a player who means well, not trying to mock, and would like to show appreciation for a in your mind good game. Taking the time to type out,That was a good game, have a nice day. Would go a lot further then simply saying gg.

I had a terrible one today: the guy typed gg whilst the game was still going on! I can only translate this as "I've won, now resign you idiot."

It is a breach of etiquette for the winner to initiate a handshake OTB, why would it be proper for the winner to say "handshake" on line? It is really simple, let the loser make the first move. While the loser might be considered less than a great sport for not offering his hand, he did just lose and should be given some leeway. It is not proper for the winner to initiate any post-game comments or gestures. If you feel your personal code of conduct, trumps any feeling of empathy or consideration for the loser; then you are arrogant.
Supplement: If the loser OTB isn't forthcoming with a handshake, I have no problem with the winner initiating a handshake after a short wait.

that's nuts
Which makes it perfect for this topic! Bunch of wackos with chips on their shoulders.
I had a terrible one today: the guy typed gg whilst the game was still going on! I can only translate this as "I've won, now resign you idiot."
Ouch, harsh one... It could be though that he was thinking you wouldn't see it after the game had finished, as it seems so few reply to chat comments. I just wrote "Great game! You play well" to a guy that beat me, as I really enjoyed the game and thought he played well, but for the aforementioned reason I wrote it a couple of moves before the end. Also, the chat function is really tiny and easy to miss.
I often write "gg" after a good game regardless of who wins, but only once have a got a reply. Then again I've just joined, only played 50 games or so. Though I agree with what some has said about "gg" being a bit informal and lazy.

I had a terrible one today: the guy typed gg whilst the game was still going on! I can only translate this as "I've won, now resign you idiot."
I say 'good game' a few moves before I'm going to resign, to make sure my opponent gets a chance to see it.
Theres an apocryphal story about a Correspondence player who sent the conditional move 'If 20.. Resigns - thanks for the game....

I had a terrible one today: the guy typed gg whilst the game was still going on! I can only translate this as "I've won, now resign you idiot."
I've only gotten that once, and with the board position it was clear that he did think I had just given him the win with my move just before he typed it, but he overlooked an out for me that I had seen. 16 moves later I claimed a draw by repetition. I thought about typing "gg" back at him at that point, but I just left the chat window blank. He knew I was thinking it.
gg for me is the same as a hand shake following the game. i guess I need to change to TFG. Perhaps someone will be offended by that too.