This is the internet. You can't know someone's intent based off of a simple "gg" or leaving the game immediately after it ends. It could be they have limited time to play so they are rushing to the next game or they simply don't want to communicate with the person they just lost to or beat, or a thousand other reasons.
The only variable of which you have control is yourself. You can create your own code of ethics ie. "always say gg, thanks for the game" or whatever. But don't expect the same from everyone else. Better yet, don't expect ANYTHING from anyone else. Live your life, enjoy the game (or don't) and consider the fact that the internet is a lot different than a table and cards.
On topic: When I say gg early and leave immediately, it's because I assume my opponent sees that they have the victory. (16 coins, 2 buys and 2 province remaining for example). So I say gg and leave because I already know the result and I consider my time valuable. If my opponent takes offense, too bad! I am already starting my next game while they wallow in their insecurity.
Sometimes I will stay and exchange pleasantries because I'm in the mood for it. Usually when it was a very close game and well played on both sides.