Thought about making a thread about this - nice thread idea.
I think Gai still has a trump card. Whatever it is, it is going to effectively kill him - essentially an attack that uses up his remaining body or life force.
Maybe something that burns up his physical body and he becomes a mass of energy for one final attack. Maybe he uses his tremendous force to create some sort of vacuum or black hole/void that traps Madara and stalls him for a while. (Just an idea, trying to be creative).
Based on his sacrifice, its likely that Gai will die in a blaze of glory, rather than slowly fade away spamming evening elephant until Naruto shows up. So, regardless of how it manifests itself, I think you're correct - Gai has still got at least one move left in the arsenal.
Nice thread!
Cake