Come on ! King's gambit is not refuted ! If there were such a forced loss for black, GMs wouldn't play it, don't be ridiculous !
Very few GM's play this line. I checked my database with over 1 million games, 365 Chess, and ChessGames.com
In my database with about six million games it seems that the KG has been played by Carlsen, Nakamura, Ivanchuk, Morozevich, Grischuk, Kasparov, Kamsky, Shirov, Short, Polgar, and many more.
You forgot to add "with very rare success."
According to Chessbase 11's reference bar, it seems that the KG has been played over 16000 times with an overall percentage of 54.8 percent, played by some of the best GMs in the world.
Dunno, but I don't qualify 50% as rare...
Either Kamsky blundered or his opponent was higher rated.
Neither posibility justifies the gambit.
You feel you are qualified to make such a statement? No insult intended, but I wonder if you understand what "gambit" means.
A gambit is a risk ... you invest material for compensation, then try to turn the imbalance into a winning position. There are no "winning" gambits, because that would imply no risk -- they would not be called gambits, they would be called forced wins.
In your opinion then, what would justify a gambit?
It is a gambit because you temporarily give up the exchange in the opening.
The Queen's Gambit is certainly not risky.