He might be the greatest, but it is that age old argument about different generations and different conditions and different game style.
Certainly, Gordie Howe would have to be on the list right there with Gretzky.
This is no different than the Micheal Jordan as Greatest basketball player while Bill Russell, Kareem Jabbar, Larry Bird and even now some say Kobe Bryant.
In football you have the same arguments between eras as you also do in baseball.
I would say the very best are in a short list class by themselves.