Well my point still stands that in 98 Gretzky wasn't remotely close to as good as Forsberg. Also I dare you to find me another player who lead the league in points, and was a minus player. No one who scored as much as he did (from 93 on), regardless of what team they were on, was a minus player period, let alone -25.
Well, the guy in your avatar was a -25 in just 67 games in a year was right there with Forsberg for the PPG lead. If he played more games, he could have conceivably won the Art Ross with a -35.
What does that say? Not a whole lot. He logged a ton of ice-time on an awful team, that's pretty much it. A couple of years earlier, he was on a stacked team and put up +15 in just 43 games.
-11 on that awful Rangers team wasn't even that bad. Lafontaine was -16 in just 67 games, Kovalev was -23, Leetch was an abominable -36.
Gretzky went from a +12 to a -11 in one year. What happened, did his play slip? Not at all. +/- is team stat as much as anything.
Forsberg was always on a stacked team with Patrick Roy. Of course his plus-minus was going to be great. Post-prime Gretzky had... Kelly Hrudey, Byron Dafoe, Dan Cloutier. Or a dizzy Mike Richter.