Full intel is what makes this interesting. Bunshin feints defeat any blitzing opportunities. Gai hitting a raiton kagebunshin would be interesting, not sure if he could shrug off the paralyzing effect of that if he decided to get up close and personal rather than going for Asa Kujaku or Hirudora.
Kakashi, I imagine, would start with dropping some smoke bombs, ala what he did vs Pain to give him some cover. He'll have time to do this as Gai would have to take the time to open the gates, which allows him to set up his bunshin strategies. Full intel works for Gai as well obviously, so can definitely argue he's going to try to avoid hitting the bunshins with direct contact, but he can't afford to spam Asa Kujaku or Hirudora. He's got incredible stamina, but it would eventually wear out, plus he has to worry about a kamui snipe from a distance.
Could argue Kakashi kamui snipes right at the start, but there's hardly any fun in debating that, and that's not generally in character for him.
I dunno, would be a fun matchup to watch though. Brain vs brawn at it's finest.