I'd say it depends on what GOAT means to you .
To me, Brady would have to be the best in any era of football you put him in and that is tough to say that he would have been the best in the 70's outdoing Staubach, Bradshaw, etc.
Bradshaw won 4 in 6 years and the Steelers won decisively in those days, not down to the wire where it could have gone either way.
Montana won big too in SF and never had an interception in the SB. He was clutch.
Brady is up there don't get me wrong, but I don't see him being better than the others. Peyton Manning would probably have won as many if he was on the Pats all those years instead of Brady.
And you can't say that Brady only had Moss for a year and everyone else sucked, because time will tell about Welker, Amendola, Gronkowski, etc. They are all good receivers in their role under Bellicheks system which doesn't depend on the big "star" receiver.
It is all relative, different eras, times, the way the game was played decades ago to now.