As the title says, could he have helped Kakashi, Sakura, Sasuke and Naruto against the great battle of Kaguya if he was fully healed and Kakashi managed to bring him in Sasuke's Susano'o before the IT took effect?
8G Gai >= Single Rinne Pre Shinju absorption Madara. Double Rinne Shinju absorbed Madara could have changed the outcome horribly for Gai in that fight, Madara could have rendered all of the attacks made by Gai useless or possibly even kill Gai in a matter of minutes. Kaguya who is far stronger than Madara's final form (Power+Chakra wise not intelligence=experience) would render Gai useless in that fight unless an opening is made for him by Naruto and Sasuke.