Well, Gai in base was keeping up with Obito in taijutsu, who had earlier shat on KM Naruto, who was parrying with Itachi earlier. So if this was just a CQC battle Gai definitely takes this. But he can't take itachi on just CQC while Itachi had ninjutsu and genjutsu allowed.
Abc logic at it's finest!
Obito was not worried about GAi or naruto in taijutsu and even stated I can read your movements. Itachi>>>>obito in taijutsu it's not even close! Itachi and naruto were not equal nor close in cqc itachi no diff was reacting to both bee and naruto while talking casually.
Next; GAi>>>naruto in cqc so you can't compare them as well since fighting styles are completely different. This abc logic y'all use is getting worse.
Itachi should have no problems reading Gaia movements, GAi will not get caught in genjutsu, GAi would be able to react to itachis speed, same for itachi with Gais speed, but the ninjutsu and clones is what might get GAi in the end. Buttttt if need be, GAi can break all 3 tomoe genjutsu so with tsukuyomi gone he wouldn't have to look at the feet the entire time. Thus making it easier on him to combat.
Itachi high diff