This isn't something that is easily solved unless it were to actually happen.
Both are "progressive fighters", but neither will hesitate to pull output the big guns if they know what they're up against. Gai can only even afford to get hit once by Amaterasu, but Itachi has to be careful not to take any hits from Gai. It's all well and good Gai using the not-look-at-the-eyes method, but look what that did for Naruto.
On the flip side, Itachi was well aware of Gai's capabilities and warned Kisame against underestimating him so is he going to fight slowly? Maybe not. At the end of the day, it comes to who "progresses" quicker through their arsenal.
Both are "progressive fighters", but neither will hesitate to pull output the big guns if they know what they're up against. Gai can only even afford to get hit once by Amaterasu, but Itachi has to be careful not to take any hits from Gai. It's all well and good Gai using the not-look-at-the-eyes method, but look what that did for Naruto.
On the flip side, Itachi was well aware of Gai's capabilities and warned Kisame against underestimating him so is he going to fight slowly? Maybe not. At the end of the day, it comes to who "progresses" quicker through their arsenal.