Amarande: This is one of those mistakes that is difficult to avoid, because difficult to actually see the consequences of until it is too late. Even computers can have difficulty seeing this, oddly analyzing the position after 27 ... e4 up through 31 Rhe2 but failing to see that this is a significant White advantage. (Yet strangely enough if one starts the analysis a couple moves later, say after 30 Qxe4+, the same computers will easily see the +1 or so advantage in White's favor at that point.)
Probably Gunston expected 29 fxe4?, which is bad and allows Black a significant edge after 29 ... fxg4 (which seems to win the Exchange at least because it severs defense to White's exposed Knight and also threatens ... f3. White cannot parry both threats).
Note that it's also all a matter of timing, as ... e4 at different moments was perfectly alright, particularly on move 20 (instead of 20 ... f4) where it leads definitely to a comfortable advantage (White has to exchange the Pawns, and then Black has a clear central superiority plus strong King-side attack chances with the open g-file coupled with pressure on h3, between the Pawn exchange unblocking the QB and the Queen being free to move to h4 as soon as White's Knight moves).
The finale is imperfect, probably due to time pressure, although this hardly explains some comments I've read, such as Ed. Lasker's comment to 33 ... Qc4. Lasker contends that Black could not have played 33 ... Bd5 on account of 34 Qd4; while indeed, 34 Qd4 is White's specific against that move, the comment is still spurious as Qc4 is far, far worse (Qc4's evaluation becomes +11.00 and higher within seconds, while Bd5 evaluates around a "mere" +4.03).
(FWIW, Black's best at that stage is apparently 33 ... Nxf3, which appears to allow him to escape into an endgame, although one which he should still lose with best play.)
What I really don't get the point of, though, was 34 Nf6+; while it does not throw away the win, 34 R1e7+ at once is obvious and even more beautiful than Griffith's ending: 34 R1e7+ Bxe7 35 Rxe7+ Kh8 and now the stellar 36 Nf6!! is the win. Black can capture neither Knight nor Queen, and instead must give up his own Queen to prevent mate by Rh7!