He isn't close to being the GOAT.
Brady OTOH already has a solid case as the GOAT.
WHEN Brady wins his next SB he will HANDS DOWN be the GOAT.
He will has a 77.5 win % in the regular season, 5 SB appearances (ie Conference Championships) with 3 SB wins, he 17 playoff wins. 7 appearances in the Conference Championship, 11 division championships.
Montana OTOH has a 71.3 win % in the reg season, 16 playoff wins, he has 4 SB sins, 4 conference championships, and 5 Conference Championship appearances .
Manning OTOH has a 69.6 winning %, a losing record in the playoffs. certainly a SB win will give him a second win and get him over 50% record in the playoffs, but he isn't in the Brady Montana GOAT conversation.