I use Good Game to mean just that; I had a Good Game.
It doesn't matter if I came in 1st, 2nd or whatever... so long as I felt the game went well.
I don't use "gg" when I've been thrashed. Let's say, for the sake of argument, that we were on a game where you had to capture 10 objectives to win. If I had 8 at the end of the game, and most players had around 8-9 as well, I'd call that a good game.
If the other guy gets all 10 and I only have 3, I wouldn't "gg" that game.
I do sometimes gg in games where I have absolutely slaughtered the opposition, but only if I feel like I really had to work to do it. For example, on a FPS; if I spend most of the game on virtually no health because I keep getting into shootouts, but keep winning them, I'd still consider that a good game - I'm constantly under threat, and thus having to work for my victory.
Far more often, I actually try and claim a Moral Victory post-battle. Usual MV objectives include;
> Having the most Kills / Experience at the end of the match.
> Earning ribbons / medals / in-game awards during the match.
> Doing better than a certain player (or group of players).
> Being the best player on the losing team (if I'm on the winning team, I don't really need an excuse).
> Being the best player overall.
As far as I'm concerned, I haven't "lost" if my name is top of the scoreboard post-battle.