Meh I guess ill give my Input as well. Nothing to serious though, not trying to get turnt up.
Regarding Gaara taking flight to sky Immediately as you say he would due note Itachi has shown Noteworthy jump height & speed in regards to the duration in which he jumps to reach his destination[
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][
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], So going by that we shouldnt rule out the factor of Itachi reaching gaara of course at a
reasonable height and use genjutsu. Now let's say gaara takes to the skies and Itachi is left on the ground? What does gaara do? Gaara's style is piling his opponent with sand and sneak attacks show cased on kimimaro and Deidara which both of them were able to counter and react to Gaara's sand sneak attacks.
Kimimaro
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Deidara
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Both capable of reacting to gaara's sand, Of course im not downplaying gaara by any means but by having the sharigan this will only make the process easier on Itachi in regards to evading his sand attacks.
Regarding Itachi reaching gaara to trap him into a genjutsu, how can Itachi pull this off? Well this is a very overlooked aspect of Itachi's arsenal which is the
utilization of crows. As shown
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which would proceed to fly up to where gaara is located on his command and then proceed to form into Itachi himself as shown in the manga Itachi is capable of using his crow clones with clones to lessen the usage of chakra. The crow clones are capable of using genjutsu,
dusk crow genjutsu to be exact as shown against naruto and sasuke during their confrontation, that being said the crows can easily or should be capable of forming into Itachi and then proceed to use genjutsu on gaara.
Another interesting detail I brought up before was that the crows have been shown to regroup into a larger form
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. Logically this can be a exploit for Itachi in regards to hopping on to the crow after it reaches a large size and ride upon it as it flies to reach gaara. The crow should be capable of carrying itachi.