Gai can counter tysukiyomi with his ability to fight while looking at the feet. But if he does that then itachi will hit his ass with Amaterasu. And then if itachi pulls up susanoo is game over. So itachi sadly
Guy starts taking about youth and shit and then itachi commits suicide. OR
Itachi does a little funny face, guy accidentally looks next thing you know guy laughs so hard he blows up.