It is strange, since many other options seem available: e6 is the first one that comes into my mind, and after the exchanges the position looks balanced (cxd5, exd5, with both bishops's lines opened for black. The problem is that g6 looks useless, since the black's dsb is pressing against c5, but if white defends it with b4, black goes Bg8, with a good-looking diagonal open).

I am very concerned with your main question "where did I go wrong", since it happened to me, and it's very frustrating. First: don't look to the database ONLY. You have to make your own evaluation of the position. Playing good moves without a plan is not good. Second: don't trust too much GK analysis's engine. Where it cames from? I observed that it came to be a little questionable when it comes to "tactic melées". Now: with this move, you indeed set a tactics, based on the queen overload. The black queen is defending c7 from a knight fork, and Nf6. If white takes Nf6, black "must" retake with the queen, allowing the fork, as you correctly spotted. But the opposite is also true: if white goes with the fork first, black "must" take with the queen, allowing BxNf6, that threatens the rook, and makes black lose the kingside castling rights, when the queenside is open. So, which is the best outcome? To take a rook with a knight on the corner is always risky: probably you won't be able to save the knight, and it's very delicate to see if the tempoes that the opponent uses in taking it and the 2 points advantage it gives you are really "free of charge". In this case, they certainly are not. My question is: did you think about Nc7 first? Moreover: tactics are good, but a good concept for attack is "build pressure". White must think about several ways of "building pressure". One could be Qa4. You can say that black covers it with Bd7, and it is probably right, but... did you think on it? White still has BxN, but also Rd1, and Nd6+. Were you sure all this came to nothing? Ok, now, let's see black's perspective. Black can defend all of the threats with the simple Be7. It releases the pin, defends the knight on f6 and allows the queen to keep her eye on f7. But black is not a passive player, and he already has the center, while your king is unsafe... he is hoping for you to exchange your dsb just to attack you with Qa5+! He wants you to move your knight b5 so he can play e4, breaking early your weak center (no matter you just take it, you're far from castling and his lsb can go quickly to g5... generally speaking, of course). So black didn't defend, and played a tricky move that makes you think he overlooked your threat, but it fact you did exactly what he was hoping for. His was a strong developing move that brought his dsb to a very nice position, and he calculated that your hunger would get rid of all your developement and threats (knight and bishop), leaving you resourceless agains his strong attack. So, my answer to your question: you simply fell into a trick he set to you. This is the main reason you lost...  

The engine doesn't know you were going to fall into a trap, so it gives an evaluation based on some other move (I didn't see). Forget it. Now Qa5+, followed with Bb4 is a real threat for black, and white must think about Qa4 or something (Ne2, in order to develop and looking for some piece exchange in order to clear up a little the board, that is becoming very "blacky"). White cannot attack black's unprotected e5 easily, and now NxN followed by Qe2 is not possible because of Bxd4, with a wonderful position... so white is already in troubles here! And the solution is not to wipe off your pieces for a useless rook... Again, I would have thought about Nc7+, Qxc7, BxNf6, Rmoves (or goes for Qa5+!), but still is not all that good for white since you cannot take e5. Can you see it? Ok, maybe I am all wrong, but the point is that I think white IS inferior now, in fact, doesn't matter what GK engine says.

And the black queen joins the assault battery, with a very nice outpost on f6.

And here, NxN was probably the less bad move, just to clear the board of some black pieces. After BxN, white has to calculate if he has time to defend b2 or if he has to take care inmediately of black's e4 (if Rb1, e4!) with an inmediate Bd3 or maybe Ne2. White has to evaluate that black's taking b2 is really a good move, since it allows Bc3+ on the next move... so for white the problem is to find a good defence.