I don't really agree.
Like a previous poster said, if the game is obviously finished, I tend to say GG just to tell the other player that I appreciated the game and the efforts that both players put into it.
I have to disagree.
If the winner wants show his appreciation for the other players efforts he should say WP alone, that's a kind compliment.
Both players know that the winner usually had a good game, so there's no need to say it first. It's like braging about it, like an anoying childhood friend.
And I think GG should mean "Had a Good Game". Losers don't have to say it. If he outplays me, yeah GG maybe a WP if he did something really clever. If it was a terrible experience for me there's no need to lie that I had a "Good Game".
It's sort of a handshake really. Forcing it is rude I think, no matter the reason.