nagato sees what his paths see,meaning there is a connection with his sense of sight and theirs,he controls them with chakra,chakra rods are just a medium,gaara connects the eye to his nerve with chakra too.Nagato's paths weren't connected to his optical nerve as he was only controlling them via chakra rods, so that's irrelevant. You're missing the point. "When Genjutsu can affect sand", how about no. Gaara shouldn't be able to possess the ability of vision through the utilization of sand and yet he does. Your thinking is limited. The eye is connected to his optic nerve which is why he is able to see and he is able to see with a Santon eye that's connected to his own eye and therefore brain so Genjutsu can affect him.
Lol @ bolded,he shouldnt be able to make a perfect replica of onoki down to his skin tone yet he can,this is a fictional manga where supernatural things occur lmao. It dosent matter what the eye is connected to,the eye is a fake copy made of sand,the medium through which sharingan genjutsu occurs is through the actual eye,hence direct eye contact is needed.
The medium through which itachi makes eye contact with gaara is through an eye made of sand,itachi cant genjutsu a sand eye,therefore he cant genjutsu gaara. If gaara actually plucked out his own eye and used it,then you would have a point.