The thing about Gretzky was the way he played required him to be laterally agile, and shifty as hell.
Think about what Patrick Kane would play like if you added 30lbs of muscle on him. I don't think it's quite as beneficial as you think given the type of game Gretzky played.
Building muscle, especially weight training, is beneficial for nearly all athletes. The attitude that being muscular "will slow elite athletes down" went out the window in the late 70s, as better conditioned athletes always excel better, than ones who avoid building their bodies. You may be thinking of stereotypical bodybuilders, who inject themselves full of steroids.
Two examples of better conditioned athletes at their time, were Pavel Bure and Teemu Selanne, who both had tree-trunks as legs. Selanne remarked during his first season with the Jets, that he could not believe how cavalier the team took training, and how out of shape players were when returning from the off season.
The game has evolved so much, especially when Europeans came over and started to dominate in the 90s, that todays players are more muscular, faster, and take their diet far more serious. Gone are the days of players like Guy Lafleur smoking a pack a day, in between periods, then going for drinks after each game.