I pondered this lots and Gretzky is still and for the foreseeable future will be the GOAT, having said that if you stuck him into today’s NHL he’d be a decent player at best, the game has changed so much and with better equipment, training, even dedication to the game (in the 50s even the stars had second jobs) players just become better and better.
Throw Gretzky, McDavid, or any future star back into Gordie Howes days and they’d be retired with an injury probably first or second game, it was a different game back then.
What I believe makes players great is how far ahead of other players in their era they are, you can’t compare past, present or future.
McDavid is showing his dominance and will be one of the best but Gretzky was so far ahead of everyone else in his time for now there is no threat to his status of GOAT.