Like in any sport, the past greats get left behind in history. They get their time, until they no longer are relevant to modern day other than as pages in history, because everything has changed so much. Same thing is going to happen to Gretzky. It's been... what, 20 years, since his retirement. While there are still a lot of people around, who remember his dominance and for whom he was the undisputed greatest ever, there are also already a lot of hockey fans who never saw him play. It can be argued that no one could dominate these days like Gretzky did in the 80s, not even Gretzky, which is probably true, but it's also true the separation to their peers players like Gretzky and Lemieux had was absolutely crazy no matter how you spin it. What makes Gretzky even more special is he did what he did without being extraordinary physically, though I recall there was an article where it was said he recovered during games faster than any/most players of his time, meaning during 3rd periods he was in beast mode since he was so fresh compared to others. Maybe that was his extraordinary physical talent, though on the other hand I'm pretty sure it was at least partly just the upside of being a relatively small and lean guy with not that much fat nor muscle
Anyway... there are only handful of guys left in the NHL who ever played with or against Gretzky. Cullen and Chara at least come to mind, I'm sure there are some others still. After none of his peers are left in the game, each day the comparison to modern day players becomes less and less meaningful or relevant. Before not too long, bringing up Gretzky as the greatest will be like someone in the present day digging up some 1920s legend. One thing that makes hockey legends age faster than some other sports is the equipment side of things. A football (soccer) player or track and field athlete looks essentially the same as 50 years ago, while a hockey player from 50 years ago with his equipment looks like a total relic from a time long past.
In my childhood, Gretzky was the undisputed hockey god and greatest player, and very well known in Finland because Kurri happened to be his sidekick (and a great scorer and two-way player by himself!). So combining his career achievements and stats with my childhood nostalgia, it's no surprise it's very easy for me to find him the greatest ever. But who am I to ask people, who weren't even born back when Gretzky still played hockey, to pick him as undisputed greatest ever and buy into the whole legend. For them, they grew up watching completely different players, those were their hockey heroes and when you put a stat sheet from the past against your childhood heroes, your brain might acknowledge Gretzky as the greatest ever, but your heart will disagree.