A Clever KGD

This game by Isidor Gunsberg was published in the November 1907 issue of "Chess Amateur."  The game is against an unknown opponent and the venue isn't given.  White undoubtedly had little respect for Black's skill and played some suspect, though incredibly clever, moves.  Black finally succumbs.

to me it seems that 6..h6 was a crucial loss of tempo. I don't use computers because I want to see how strong player can find the good moves in all kind of positions. In KG accepted and also declined, the loss of only one tempo may be really losing mistake. Since at first sight the black defence after taking the queen seems forced, the losing mistake might really be 6..h6.

The thing about such a nebulous idea as loss of tempo is that it's only crucial if the other party knows, or can find a way, to take advantage of it.  Few people have the ability to do so in an exquisite manner.

Where is the mate if Black simply played 12...Bxc2? 

It seems this was a completely unsound sacrifice. If so, it takes away from its intrinsic beauty. This isn't like the Immortal Game where generations of masters have analysed the game and found some subtle improvements. This is just busted.

Which is why I noted the "suspect" moves and Gunsberg's lack of respect for the skill of his opponent (who had no access to computer analysis).  The moves were clever and difficult for his opponent to counter.  Odds-games are basically "busted" from the start, but many are still clever and exquisite.