Itachi has these advantages over Gai:
-If Gai is busy looking at Itachi's feet to battle, he would be prone to Amaterasu because he can't predict it coming without looking at his eyes.
-Unless Gai knows this, Itachi also uses his finger to initiate Genjutsu.
-Susanoo has powerful defense with Yata Mirror and Gai would have to be aware of its use along with Sword of Totsuka.
Gai has these advantages over Itachi:
-Gai is super fast on his feet, meaning he would be able to dodge most of his direct attacks.
-Asa Kujaku is also long range too I believe, because the friction in the air is fired in a large arc forward. We see this when Gai fights Kisame for the last time against all 1000 of his sharks, his attack had to be long range because it needed to cover long distance and wide area. I could be mistaken so dont take it 100%, but if its medium-long range its a powerful move.
-Hiru Dora is also an immensely powerful attack that could damage Susanoo directly if its not completely transformed.
-In 6 Gates form, it doesnt completely wear him out as fast as we believe. Because he is currently using it against Tobi and his Paths of Pain. So with enhanced Taijutsu, all he needs is to connect one combo.
Verdict: I really don't see Gai getting past Yata Mirror, unless Hiru Dora is that powerful. But we havent had that proven yet, so we assume it cant. Gai should be able to manuever the Sword of Totsuka and maybe try to wear out Itachi instead. But my vote goes to Itachi since Gai is very underhanded in this battle trying to avoid eye contact.