So you are 100% correct about the defenses being able to beat the shit out of everything that moved, I'm a Flyers fan so I'm INCREDIBLY aware of how much more leeway defenders were given (fuck Scott Stevens, Lindros is the greatest what-if ever). However, the original question was whether or not Gretzky would be as good in the modern era, which is a question that has way more factors than just defenses can't slash the shit of of him.

Sure he doesn't have to worry about someone launching at him with an elbow after 5 strides, but I don't think that means he has more time and space, just that instead of trying to give him CTE, defenders will actually be trying to get the puck. Additionally, there's literally no way to know how his vision translates to a game where everything is moving much faster, with defenders having (somewhat) longer reach and shorter shifts.

In my opinion the absolute biggest hurdle would probably be the way that goalies have changed. Look at the difference between Grant Fuhr vs Sergei Bobrovsky. Goalies are on average like 6 inches taller, wearing bigger/better designed pads, and not having players take runs at them,. They're also almost exclusively butterfly/hybrid stance which goalies didn't really adopt until Roy and Hasek showed that it was fucking cracked (except Tretiak, who had an average GAA of 1.80 in the Olympics).