Here are some rules of etiquette I have for online chess:
1. If both opponents are from English speaking countries and the loser says gg, then the winner should reply gg. To not do so is a sign of disrespect. If the winner says gg, the loser may choose to either say gg or not respond.
2. If the loser offers a rematch, the winner should try to accept if she or he has enough time. If the winner offers a rematch, the loser is free to choose to accept or reject it.
3. It is not proper to log off the website to avoid losing a game. It is understandable if your Internet connection breaks during the game. The losing party should either resign or play out the game.