Genjutsu doesn't work on gaara (atleast eye genjutsu)
But not how u state it...
It doesn't work cuz gaara can make artificial eyes out of sand ppl hav said ppl lik itachi can still get gaara cuz it's still attached to nerves well thts false for 2 reasons one its artificial and the next was in fight with mizukage mu
He had eye wandering around and both of his eyes were open
Now anyway genjutsu is usually to try to subdue your opponent so u can knock him out physically while he's trapped mentally (exceptions kurinai itachi )
With tht being said if somebody some hoe managed to get in range of gaara (0.0) and cast it but then tried to hit him phyiscally the sand by feats due to his mother will would still defend him I think thts what the OT meant tht gaara is impervious to genjutsu
I think sound genjutsu can get him tho
Although not sure about if he goes inside his ball