He has to stand in one place to open the eight gates.... so kakashi wont let that happen...Kakashi has better Taijutsu,Ninjutsu,and Genjutsu with Kamui.....Even if he does stand still and starts to open the eights gates kakashi will just use Kamui to rip his head off...lets see him heal from that
Don't be a moronic fanboy, please. We know Kakashi is good but he's not that good.
Kakashi has ninjutsu and genjutsu going for him but Gai is a total beast with his taijutsu and he's trained specifically to defend against genjutsu and the sharingan. Gai does not have to stand still to open the gates, he has been seen doing it while moving, and kamui only works if Kakashi can focus on his target, which wouldn't be an option since Gai moves so damn fast.
I say they're pretty evenly matched, and unless one of them screwed up they would just knock each other out.
Ohh, and also, using kamui has adverse effects that aren't thoroughly known. It may cause blindness, but even if it didn't it uses up absurd amounts of chakra. Kakashi wouldn't be so reckless as to use it unless totally necessary. If he used kamui and missed, then Gai would rip his head of.