That's all Gai has a counter for? Genjutsu is what most users reply on when dealing with Itachi
because itachi's a genjutsu fighter. It's like removing mizukage's clam, or shi's gemjutsu because "hur dur that's all people say".
Anyway, Itachi wins. He and guy are both the only people with 5 in speed(so far in db). That means, outside gates, guy has no speed advantage. Ontop of that, itachi has the sharingan and has shown feats of parrying highly skilled cqc fighters. Guy's not overwhelming itachi in the slightest.
Itachi kills guy with a combination of shuriken and bushin.
Or, itachi 1 shots guy with amaterasu.
Or, itachi opens V4 with yata and blocks everything guy has to throw and solo's guy with totsuka or magatamas
Or, itachi kills guy by outsmarting him and getting him itno a situation where a katon kills him.
Or, depending on the location, itachi kills him with suiton.
Take your pick. Any of those kill guy. The only offence guy has is gate 6+ but v4 with yata would tank that for the few seconds it lasts.