He played the greatest game of his entire career just before the SCF began.
Not even close to his greatest game. Probably around 32nd. (If we're talking just playoffs, maybe it's around 12th or something...).
He was not even close to top form. His late-prime, after 13 pro seasons, had ended two years prior to the '93 Cup run. His peak was roughly 1981 to 1987.
Why couldn't he lift the Kings to a win over the Habs?
Well, he'd already lifted the Kings to wins over Calgary, Vancouver, and Toronto, all of which were significantly better teams than L.A. that season.
The Kings were better in April than Calgary. They were also better than Vancouver in late April (the Vancouver series was likely the best series the Kings ever had while Gretzky was there). I don't really think they were better than Toronto in that seven-game series -- the Leafs actually played better. But sometimes Gretzky happens (game 7).
Gretzky was also great in game 1 at Montreal, and pretty good in game 2. The Kings were in control of game 2 and were likely to win, taking a 2-0 series lead back to L.A., when the McSorley-stick thing happened. That's when the clock struck midnight on the Kings. They lost three games in a row in overtime.
As a sign of how much the Kings played over their heads in '93, consider that the very next season they were a .390 team that missed the playoffs.
Their defense was atrocious; goaltending average.