I ONLY say gg when winning and I make sure to say it the second that I know he cannot possibly win (less than a 1% chance of him winning, and would take several large errors on my part to lose) The sooner this happens the better because most of the time my premature gg results in them leaving the match. Like little sheep they have to be told when they have lost.
If I lose I get the hell out of there immediately. If they say gg I remind them that it really wasn't, that I am the superior talent, and that pokemon by nature has a certain degree of luck to it that can make it seem that someone who is more talented was actually beaten, but the real winner was the games mechanics. I congratulate them for being part of the 1 in 100 battles where there are enough haxs for them to actually win, and if they follow me/PM me after I leave (I don't stick around to give them a chance at rebuttal), I screenshot it, send it to a mod, and they are usually locked from chatting.