Three points.
With all due to respect to Gretzky, he only competed with other Canadians for the most part. There is simply too much talent in the NHL to win the Art Ross every single year.
Right now.
1. Player from Russia (didn't have any players in the NHL in the 80-ties.)
2. Player from USA (had a fraction of todays numbers)
Same goes for every other Country except Canada(which is roughly the same given the increase in number of teams) which will sometimes win the Art Ross over McDavid.
Second point is , team. Gretzky played on a stacked team. It does matter , what the team around you does.
Now having said that, McDavid is no Gretzky or Lemieux. McDavid is not producing at some level that is much above his peers,he never has and most likely never will. McDavid can be in the Crosby/Jagr category though I'm not convinced McDavid will have the longevity of someone like Jagr due to how much his game relies on speed, though I'm sure he will adapt as he ages, it's still a question mark for me.