Petrosianic: <Sastre> <I think the purpose of 28..g6 is just to defend against the threat of mate on h7. Black's knight is then not tied down to defending h7.>
Yes, but why at this moment, with the fight in the center raging, instead of an immediate Rb8? It's not clear. It's not a <bad> move, just not clear.
It seems like Black equalized fairly early here. White is maybe even just a little bit worse most of the way, but doesn't really fall apart until the end.
I think I'd prefer 32. Bb6 to 32. d6. White's passed QP should give him enough compensation to hold, but no need to give up the a pawn too easily. Especially since the d pawn starts to become a target once it hits d7. In fact, I'm not sure it can be sustained there at all.
But White may have gone off the rails even earlier. He seems to give up the b and a pawns too easily, and be putting a lot of stock in that passed d pawn that's not as dangerous as it looks. Maybe 30. Rb1 or Ra2 would be better, although both are ugly.
Black obviously thinks he's better, hence 36...Rb8, avoiding the repetition.
37. Kh1 is where White starts to come unglued. It does nothing good, just hides the King from imaginary dangers, while unprotecting the Bishop and helping set up the fork a few moves later.
38. But Ba4?? is where it all unravels. With the QP now blockaded by the d8 Rook, Black's queen is no longer tied to it. White seems to forget that.
Of course, what's the alternative by that point? 38. Bb3 Rxd7 39. Rxd7 Qxd7 40. Qxd7 Nxd7 and White is two pawns down, but with some compensation from the two Bishops. White might possibly hold this, but he's in big trouble and obviously went wrong earlier.
It LOOKS like 37. Kh1 is the real stinker. If not for that, White's 38. Ba4 would have been playable. As long as he can hang on to the d7 pawn, White's not going to lose. The problem is I'm not sure he can, and Black's c pawn looks more dangerous than White's d. It's quite possible that White was lost much earlier than he appears to be, as soon as he started shedding pawns and putting all his money on the d pawn. Even an engine probably wouldn't help you much with this, and might show Black only marginally better even if he's dead won.