Kakuzu has powerfull atacks and can atack from all sides but Gai has largelly the speed to dodge them all ( Kakashi barelly handled it even when Kakuzu had Hidan and when he had to babysitting the team of Shikamaru).
Kakuzu is very good at taijutsu because of his tentacle but against somoene with the strengh of Gai it won't work.
i wonder how Kakuzu will touch Gai, that's why i don't see him winning.Gai's speed and strengh is very bad for Kakuzu because speed and gates are the key here.
Even Kakuzu's masks are out of his body Gai with his speed can take them one by one with 5-6 th gates.
7th gates should destroy Kakuzu doton skin.
If gai have to use 8th gate then it would be a draw.
I see Gai win more than not.
Due to Doton Domu, none of Gai's attacks would cause any harm to Kakuzu. Even Hirudora, a pressured based attack he unleashed at Kisame didn't deal a terrible amount of damage to him. Someone with diamond skin would surely take the hit without sustaining much damage, if not any for that matter.
Kakuzu, would eventually outlast Gai in this match, making this a match of endurance.
Imo, Kakuzu mid-high diff.