Gai would win. He is a tai jutsu master and has excellent speed. Kakashi may have sharingan but it requires eye contact at times to under stand the moves. Gai has trained himself to not look at eyes when fighting but instead of feet which would help him big time. The only way kakashi could get gai in a sharingan genjutsu is if he looks in his eyes which he won't. His true speed is unknown and we know he can open 7 gates at least if it gets too tough. Kakashi may be able to use kamui but I doubt he has enough training to catch 7 gate gai which is when he would use it. Gai's raw power and speed alone would overwhelm most of kakashi's defensive jutsu also help dodge his offensive. Kakashi was only shown opening one gate so we can assume he can't open them all but has knowledge about it. That knowledge would be bad for gai which will make it a troublesome to fight him. But after a long and exhausting battle gai would win because of his stamina.