Gai wins this ofc when he opens the 8th gate. But that would be a one shot because there won't be a person who could heal him.
Maybe Kakashi would know a way to defeat his long time friend because he knows him so good. Kakashi "just" lost the ability to see movements better and Kamui. I don't see Kakashi using Kamui against a super fast moving target anyway. The only thing that Kakashi has is a huge arsenal of Jutsus Oo
Well, Kakashi can open the first gate. Without Sharingan though, Gai shits on him. 6th gate neg diffs, 7th rotflstomps -10000 diff. 8th is too embarrassing for Kakashi to even comment on.