Itachi wins this mid-high diff.
-Itachi's speed does indeed surpass Gai's despite common belief of Gai's speed being on another league when compared to Itachi's.
-Itachi's intellect is enough to outsmart Gai ten times over in this fight. Exploding Bunshins will not be easy for Gai to read through, nor will he read through the crow clones. This will cause trouble for Gai as not only will he not anticipate them (Sasuke with a developed 3T couldn't see it due to Iachi's hand seal speed) but he will also be countered with their use.
-Genjutsu will eventually catch Gai off guard despite his ability to fight by looking at the feat. Realizing the fact he looks at the feet whilst fighting, Itachi can use a crow for genjutsu, which will likely be the ends of Gai. He may break out, but that doesn't change the fact he have stopped looking at Itachi's feet and looked at either Itachi's fingers or Itachi's eyes, accidentally of course.
-Fire release enhanced kunai's will likely be a problem to Gai, someone who usually needs to either block with his strength or evade. However, evading isn't as easy as it seems considering Itachi does it while air-borne, meaning he will no know where they are coming from, since he would need to look up, a perfect opportunity for genjutsu.
-Gai's movements are effortlessly read, so his taijutsu and linear fighting style won't pose much of a problem since he kept up with KCM Naruto with ease.
How does Gai win this again, taijutsu kicks and punches won't end this.