the question i would ask is the opponent qualified to determine whether the game was "good" or not ?
Nobody says "Good Game" anymore!

I use focus mode and have not figured out how to chat (text) yet in it....plus I really don't want to during the game, I need to concentrate....I am very social though when I play OTB....there's just something about the internet that makes it impersonal to me.

if someone says good game, or, gg, it should be met with similar, IMO. It doesn't matter whether it was a very good match or not since this is really just about an online handshake. Say gg whether you win or lose. Either way is appropriate;. When someone wins or loses, saying good game is simply thanking your opponent. And, thanks for the game is not better because it belittles the opponents match. Good game is best, and thank you is secondary because they didn't do you a favor by playing you.
I say gg sometimes when it was actually an interesting game. I feel just randomly saying gg to some guy you just crushed is actually a bit snobbish as it was not a good game.

If I said GG every game I played I'd be like Crocodile Dundee saying 'g day' to everyone as I walked around New York City

i remember i lost something like 4 times by someone, when i asked for rematch , he said good game.... :'(

If I said GG every game I played I'd be like Crocodile Dundee saying 'g day' to everyone as I walked around New York City
Crocodile Dundee would probably say good game after every match. The guy had class. Good game is not an insult. I stand by the virtual handshake analogy; when I was young, handshaking was always appropriate after any match. It was just good sportsmanship. As an aside, I play vintage baseball and the whole team shakes each other's hands whether it was a blow out or not.

When playing over the board, I was instructed as a child to always say good luck at the start of the game, and to always shake hands and say good game at the end of the game. I don't think this is absolutely necessary in online games, but I have noticed that almost nobody I play on chess.com ever gg's, regardless of the outcome of the game. It feels kind of cold. But whatever.
Never say GG to players who don't resign in lost positions.