Can Naruto SO6P heal Gai completely? We saw Naruto in form of having sage of the six path powers and mentioned he could do almost everything.
He did save Gai's from near death,
He heal Kakashi's eye
He slowed the process of Obito gettin rot.
With these facts, I believe SO6P Naruto could heal him, but I'm still not sure.
He could not fully heal Guy. Naruto even said this himself once he was talking to Kakashi. He said that Guy was too far gone to be healed fully. To support this, he couldn't fully heal Obito either. He just delayed his death.