You can contact me or Stef here directly (by Personal Message) or on discord, in case you want to report someone.
In this case, while swearing is of course not acceptable, your opponent might have gotten the idea that you were mocking them. You did nothing wrong, but the flow of events, when viewed by an opponent (who may not immediately see your remark *before* he had the bad hand) might be construed by them as bad form.
I personally use "GG" only when the game is explicitely over (End of Game message on screen), when I see they have the money to buy the last Province/Colony in the info bar, or when I want resign myself, which I then do immediaetly afterwards.
I've seen opponents miss an 'easy' 3-pile victory, and of course even with 10 hand cards and $7 already from Militias etc. a player might not have the last $1 to seal the deal.