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Gai moves at 300KM. Kages don't.
Even the great Jiraiya sama was one shotted by Gai. Keep in mind Gai wasn't even using his gates.
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Gai made Kisame his bitch.
Itachi's a smart character, he knew Gai would beat the hell out of him so he decide to run away.
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Don't get me started on the final gate....
8 gated Gai is not a man. He is a being which makes Rikudo sennin look average.