Oh shit, totally forgot, gonna do it now, I'm a bit tired right now, so I'm gonna make it short.
-Assuming Gaara can separate Sasuke from Susano'o without Onoki's help, Gaara needs Sasuke with his feet on land to do that, which can be easily countered by hawk summons.
-Other than removing Sasuke from Susano'o, there's literally no way Gaara is getting past Susano'o, outlasting is the only way (if he can survive long enough to do so).
-Amaterasu can be countered by blocking Sasuke's LoS with sand (or shrugging it off with Sand Armor, but that will only work once), however, that would obscure Gaara's vision as well*, which would lead to Sasuke calling out Susano'o and firing a Susano'o arrow, and considering Yasaka Magatama went right through
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, the arrow would replicate this feat with ease.
*P.S.: This drawback can be nullified with third eye, however, considering that the first thing Gaara would do after seeing Sasuke's eye bleed would be forming a sand shield to defend himself, and that Susano'o comes out with a thought, and that the arrow travels faster than someone as fast as Kakashi can't physically react, it's unlikely that the third eye would come out on time.
-If all else fails, Sasuke goes up on his hawk, uses as many Amaterasu as he can, along with a few Katon's to heat up the atmosphere, and then kills Gaara with Kirin, considering he can't evade it or block it, that's when Gaara's life ends.
I might be missing some things here, as I'm not too familiar with War Arc Gaara's abilities (I didn't really pay that much attention to this arc due to the extreme boredom) and I'm sleepy af atm.