well without knowledge, Itachi takes this simply because of tsukiyomi a simple eye contact would defeat Gaara and he wouldn't be able to break it without ms. But with knowledge, Gaara would put up a much better fight using sands to cover his eyes and anticipate Itachi's attacks. However, Itachi's intelligence would be hard for Gaara to deal with. Itachi can still outmaneuver Gaara and hit him with amaterasu/ tsukiyomi. Overall, Itachi mid to high difficulty. Despite being the kazekage, Gaara lacks the battle experience and rational thinking ability that Itachi has.