Just going to point out a few more things.
-Claiming that Gai is just going to look at Itachi's feet the entire time and win isn't going to work here. He wouldn't sense amaterasu coming and he'd get scorched.
-Gai will look at above Itachi's feet, considering that where they'll be fighting, there won't be any water so Gai cannot depend upon looking at a reflection. He has no idea that finger genjutsu exists. No matter how you look at it, he's getting caught. He'll also have to avoid looking at Itachi's crow summons'. Kabuto sealed off his vision and relied on his DSM sensory abilities in order to fight Itachi and avoid his genjutsu, just to be safe...and in the end he still got caught even while being an Uchiha tank. In all honesty, Gai isn't avoiding it here...and I'm saying this as someone who is fond of Gai's character. He's only winning this fight with 8 gates.
-He's not fast enough to dodge amaterasu. Amaterasu spawns exactly where the user's line of sight is. If Itachi is looking at Gai, then Gai is getting scorched. And if Gai is attempting to avoid eye-contact, he won't see it coming. Also, it's not like Gai will be constantly on the move...he's going to stop at some time.
-Yata will deflect Gai's attacks. Itachi can hold it up longer than when he fought Sasuke, especially considering here in this scenario, he can start with it right off the bat if he wants to. And the claim that Yata only deflects what is only right in front of the susanno is false. It expands to cover the full range of the attack.
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Also, if Gai attempts to hit that with his taijustsu and is reflected...would that hurt him in any way?
-Saying that Itachi is going to hold up his susanno aimlessly for the entire fight and that Gai just outlasts him is a pathetic argument. When has he ever done that? Madara and Sasuke are the Uchiha who start out with showy techs like susanno...Itachi is the one who starts off with the lowest thing in his arsenal, kunai and simple weapons. That's the type of shinobi he is. If he sees that susanno is not benefitting him, he's not going to keep it up.