An in character Gai isn't instantly gonna go to the 8th gate, and Itachi can defeat him in that time. He could have put down the likes of Orochimaru if it weren't for his regeneration in that time.
True, he wont use it right away but just powerscaling stats the creep is absurd
Guy would start AT LEAST 6th Gate off the bat against an opponent like Itachi. And 6th Gate can probably dance circles around Itachi, he was dancing around several V2 jinchuriki with it
Itachi would try to counter with genjutsu and clones. Clones wont work. Finger genjutsu could, and that is Itachi's best bet but theres a plausible argument to be said that Gates can counter genjutsu due to the limiters being off constantly pumping chakra and making it unstable
Amaterasu wont work, too slow. Tsukuyomi wont work, Guy will avoid eye contact. And if Susano comes out, so will 7th gate and it's Hirudora time
If we put up their best, MS vs 7th Gate, I choose 7th Gate every single time (8th gate excluded)