Outside of the desert, Gaara stands no chance at all.
In the desert, I'd give Gaara at best 50:50 odds.
Gaara can win because:
1. Gaara can hide in a flying sand ball and locate Itachi using sensor sand. Amaterasu is useless, as Gaara can simply shed an outer layer of sand, and it's off the ball. Tsukoyomi is now useless, and unless Itachi can cast it through walls, so is finger genjutsu.
2. Gaara can bury Itachi's Susano'o using a massive amount of sand from all directions. There is pretty much no way Itachi can dodge this in the desert. Yata Mirror doesn't prevent Itachi from being surrounded and buried. In addition, Gaara can secure Susano'os arms and prevent it from cutting itself out. At this point, Gaara simply waits for Itachi to run out of chakra, then crushes him when Susano'o disapears.
However:
1. Gaara can only pull this off if he manages to get into the ball at all. Itachi has demonstrated that he can end fights in an instant, and there's no guarantee that he won't simply use genjutsu, Tsukoyomi, Amaterasu, or Susano'o>Totsuka stab right away. On the other hand, having fought Sasuke and Madara, in addition to Itachi's genjutsu prowess being widely known, Gaara might be able to take countermeasures in time.
In short, even in the desert Itachi can win right away, but if for any reason he doesn't, Gaara can win without much difficulty.