He wasn't as dominant as Mario at his best.
But that doesn't mean he's not the greatest overall. Pete Sampras at the top of his game was the most dominant tennis player I ever saw but few call him the greatest tennis payer ever.
I would still put him against any other player in history when he was in his groove. Easily.
Oh dear! Just for starters, 4 200+ pt seasons, plus another 180 pt season, while Mario hit 199
once! Mario was phenomenal no doubt, but saying he was more dominant when both were in their primes is incredibly ridiculous!
OT - Also like to point out, Gretzky with a bad back in his last season put up 62 points in 70 games, during what was among the lowest scoring seasons we have ever seen in the modern game. That era included top flight goalies, like Roy, Hasek, Belfour, Cujo and many other good goalies, so not like he was scoring on Peter Ing every night or something. When he suffered his back injury he was no longer the "dominant" Gretzky he once was, and still lead the NHL in pts in 93-94 with 130 pts. He wasn't even close to the "prime" Gretzky anymore at that point.
Also, as others have mentioned, Gretzky is unmatched in terms of hockey IQ, yes that includes McDavid, and Crosby and any other player of today, and for that reason alone, he would still be a tremendous player, even if his numbers would be a bit lower. Young Gretzky in todays game, would still lead the league in points, and not just by 5 or 6 pts, but 20+ most years. People always forget this about Gretzky, he was miles ahead of the 2nd best, all the time, including those on his team, and sometimes it was double the next best, with todays training and qaulity of coaching and equipment, he would still very much be a dominant force!