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.
From what i observed it seems that all Naruto did is just preventing his death, after doing that however Guy couldn't completely recover from all the damage he received by using the 8th gate (as shown in Boruto).
