From 79/80 to 89/90 (Gretzky's 1st 40+ goal season to his last one)
Gretzky's last 40+ goal season was in 1990-91, not 1989-90.
He should have scored 40 in 1993-94. At the moment he scored his 802nd goal to pass Howe, he had 37 goals in 73 games (pace of 42 or 43 for that full season), but I think he lost his mojo after that game. The Kings were heading into the crapper and the team owner (the main reason Gretzky had gone to L.A. in the first place) was heading to prison. He just seemed to sign off after that, and actually sat out the last three games of the season (finished with 38 goals... still won the scoring title).
As far as "the trap" goes, it isn't a late-90s' invention. It goes back to the 1920s. It was out of fashion in the post-expansion era and for much of the 80s/early-90s, but prime-Gretzky did face "the trap" of sorts at times -- for example, against Calgary in 1986. The Flames' strategy in that series was to stack the blue-line, and also to force Gretzky and Coffey up the left side of the ice rather than the right. That's a little different from the conventional "trap" (which generally forces the puck carrier up the middle), but is essentially the same thing. In that regular season, Edmonton had scored 5.33 goals per game, and against Calgary in the playoffs that dropped to 3.43 per game, largely because of this system (and The Flames' very disciplined play). As for Gretzky, he scored 5 goals and 13 points in the series, so in a sense he still did very well, but most of his real damage was limited to game four. Flames won the series accordingly.
Gretzky also faced Brodeur and the Jersey-trap in the '97 playoffs (among other times). The Rangers rather handily (and surprisingly) won that series in five games, with Gretzky leading all scorers with only 5 points. But it was a low scoring series.
*********
Getting back to the thread topic: Ovechkin is about 247 goals shy now, right? Let's say he needs 235 more after this season. He'll be 34 years old at the start of next season. As I said earlier in this thread -- I think, on about page 5 or something -- the most remarkable thing about him (besides the goal numbers) is his endurance. He's just a tank that keeps going. But he has played a rough and edgy style for years (esp. when younger), and I think at some point it catches up to you, no matter how "easy" the new NHL is, physically. He may not ever get blindsided by a crushing check anymore, but the mileage has an effect on everyone, at some point. Anyway, to score 235 more goals from age 34 is extremely difficult, no matter the era.
One of the things that will determine how high a number he racks up is how good/bad/average Washington is after this season. They're a very good team right now, with an above-average power play, and some players who have chemistry with Ovechkin. But if they were to fall off the pedestal suddenly and be stuck in the lower-scoring half of teams for the rest of his career, it gets that much harder.
I just don't think it's going to happen, but I'll the first to toast him if it does!