Guy wins. He is not getting caught by Tsukuyomi or any Sharingan Genjutsu since he trained himself to fight without needing to look into his opponent's eyes, and also because doing such a thing isn't all that hard to begin with. In the small chance he gets caught in a Finger Genjutsu, he simply dispels it like he did Kabuto's high level genjutsu, whereas the pain he gets from releasing Gates would also be enough to break him out.
As for Amaterasu, if Guy was hit with it before using his Gates, then the
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of releasing either the
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or
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blasts the flames off of him, much like how
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. Whilst he's in Gate 6, he can probably dodge Amaterasu, since he was able to dodge Madara's faster-than-Kamui truthseeking balls in a lower Gate, whilst also carrying Kakashi. He can most definitely dodge it in Gate 7 if he could surprise Juubi Jin SM Madara with his speed and push him back (the same Madara who casually blitzed SM Minato so quickly he couldn't even teleport away).
Itachi cannot spam Amaterasu, not when using 2 Tsukuyomis and an Amaterasu had him panting (the same situation occurred when Sasuke used
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, so that can't be attributed to his sickness). Neither can he camp in Susano'o for the whole day, especially when just maintaining the ribcage version will put
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, so much pain that
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. Guy can pressure Itachi into using Susano'o by using Morning Peacock, then just dance around him dodging his attempts to attack with Susano'o until he tires himself out, then just launch an Afternoon Tiger in his direction when his Susano'o reverts to a lower stage. If that doesn't kill him then he would be so damaged that he would hardly be able to defend himself from another one.
