Mario "could" have been the GOAT, but we will never know for sure since so much of his prime was stolen by injuries and poor health. Just like Orr could been the GOAT if could played for more than have dozen season with a healthy body.
There are/were players with better all round games, more natural talent, and the potential to have been greater...maybe, but no one ever came close in term of objective mesurables.
When someone else gets 92 goals and 120 assists, or 2857 carrer points plus these records http://www.waynegretzky.com/statistics_records.aspx
, they can make an arugment for GOAT.
oh, and throw these on his resume http://www.waynegretzky.com/statistics_trophy.aspx