I gave an answer here already.

the thing with gai is though, his buff increases body damage caused by your team,
so optimally you would want to use ninjas with mysteries that do body damages and not ninjutsu damages.
you can check what kind of damage your ninjas cause by looking at the little yellow diamond on the top left corner of your ninjas' talents, nin means ninjutsu, and tai does body damage.

wow! thanks. i have kabuto too. sounds awesome