Definitely not a drop off until about 1991. The Suter hit.
People look at 1987 and say "What, only 183 points?" Well, a couple things here, for some reason Gretzky had 181 points with 6 games left but only got two points in those final 6 games, sitting out one of them. After the Flames in 1986 it all became about being ready for the playoffs, perhaps better rounded, better rested, etc. Put it this way, if Gretzky puts his mind to it, can he get 19 points in 6 games to get to 200? You bet. In 1987 he had 15 points in his first 4 playoff games. I can see a couple of different times at least during the 1987 season where he had 19 points or more over 6 games. So I don't think the 1987 season is a drop off at all. Plus, look at his play in the Canada Cup that year.
Even in 1988 he is on pace for 189 points or so and he had probably the best postseason of his career. No dip there. Goes to L.A. and gets 168 points on a team that was nearly last place. Yeah, no dip either, look at the context of it. 1990 was a strange year for him. 142 points was low for him at that time, even being on pace for 156 just wasn't enough. Not his type of numbers. 1991 was back to normal again. 163 points and 122 assists. I don't know if I would say he was worse in 1991 than, say, 1985, but he was focusing a lot more on playmaking than goal scoring by then. If there is a difference, it isn't that much. Perhaps a tiny dip in play just because he is in his 12th NHL season and it is darn near impossible to do what Gretzky did so often in the 200ish point level that even when he can't hit that anymore I still don't call it a knock.
But the Suter thing hurt him. Then his dad getting that brain aneurysm right after. His 1992 season was trying, that's for sure. 121 points, even missing a few games that's not just good but that was his best by then. Still was good in the mid 1990s, but as others have said, he is slightly better than Adam Oates at this time, 1996, 1997, 1998, etc. and that is a noticeable drop. I think 1999 was his final drop. He just wasn't the same anymore by then, his back was flaring up, he was terribly slow out there. I hope anyone that saw him for the first time didn't think this was the real Gretzky. It would be like wondering what all the fuss was about with Muhammad Ali when he loses to Trevor Brebick thinking that this was all there was to him.