Not only here, AFAIK (as far as I know) it’s one of the traditions on most (if not all) English-speaking Go servers.
And IMX (in my experience) wishing the opponent a good game by typing “gg” is about as popular there as elsewhere on the ’net “ty” is for “thank you”, LOL for “laughing our loud”, etc.
;) which is short for
;-) which is the emoticon (i.e. also short) for “wink”
Some, of course, write “Onegaishimasu”. Maybe we should introduce a new shortcut for this … like … “ogs”
Nope, there are no real rules, it’s just common courtesy, I’d think.
Although … I’ve always perceived “gl” to be somewhat, uhm, don't know how to put it, to me it sounds as it my opponent means that I’d NEED luck to win and then I think to myself, “let’s see who needs the luck, I’ll just play best I can”.
Sometimes, when playing a stronger player, I have said: “Let’s have a good game, please teach me.”
(And no, I don't always explain the acronyms I use )