From the Sixth Gate onwards, Guy escapes from Amaterasu and any genjutsu through sheer speed. However, Guy is only able to break through Itachi's complete Susanoo from the Seventh Gate onwards, using the Hirudora, which greatly exhausts him after its use.
He couldn't keep all the Gates open forever; at some point, he would get tired, and he heavily relies on his high speed to evade Itachi.
Itachi wouldn't stand a chance if Guy had the Eighth Gate open; the Uchiha would be defeated without even getting a proper look at Guy. The speed of the jounin in this state is absurd; even Madara Jinchuuriki could barely keep up with him.
In summary: Gai would win after opening the Seventh Gate in both scenarios, as long as he uses the Hirudora from behind the Susanoo, rendering the Yata ineffective.