Gai has a counter for every ability Itachi has
Genjutsu is beaten by speed and the chakra flaring of the Gates. Killer C clearly stated that genjutsu only works if the caster can keep up with the mental signals of their target, so if Sasuke couldn't keep up with V1 Raikage then I doubt blind Itachi can keep up with Gai here.
Chakra armour is beaten by Soshuga, which shattered boulders. If they stalemate, Gai can keep his technique open for longer than Itachi can his armour
Ninjutsu is literally blown away by sweat steam.
Feints and all that are beaten by instincts, which we saw with Lee vs Gaara.
Its only a matter of time, which may not belong considering the force that Gai could throw weapons with.