Could Guy have beaten Itachi and Kisame?

[Waltz]

No, not on his own.

 

[EliteKakashi]

Gai is Kisame's perfect counter. Kishi couldn't have made that any clearer than when he made gates have the power to repel water.

Itachi? Not without the 8th gate, but we don't know it's power.

Both at the same time? Read above.

 

[Amaterasuice]

Gai would have trouble with just Kisame, Itachi would defeat Gai with low diff.

Putting them both together vs Gai is just cruel. :/