Guy alone matches Madara.
A non-serious Madara who wanted to see Gai's techniques.
Madara didn't even use Limbo (which would have been fatal for Gai) and Gai was helped by Minato/Kakashi/Gaara/6G Lee in this fight.
Akatsuki shit diff them.
Gai need at least the 7G to defeat Kisame+Samehada. Deidara, Sasori (with the Iron Sand from the Third) can fly. Hidan is immortal, Konan is intangible, Zetsu is almost immortal and can quickly travel in the ground. Kakuzu is almost immortal and have the domu protection.
Even without taking into account Orochimaru and his puncture proof side . The Akatsuki has everything to resist Gai and Lee.
If they use the seventh gate then they're dead. They won't be able to kill the Akatsuki and then will be vulnerable (the after-effects of the seventh. Gai would have died against JJ Madara without Lee).
Even with the eighth I don't see them managing to kill everyone in time.
Anyway Gai never starts with the eighth gate and if he doesn't use the eighth one from the start, he's screwed, same with EOS Lee. Deidara bombs, Konan bombs, Kisame Suiton, Sasori Poisoned Satetsu ect... it's too much for them.