At this point, Sasuke could definately beat Gaara. In the first series, Gaara would probably have pwned him, but not in Shippuuden. Gaara couldn't beat Deidara, but Sasuke did (ok, so he BARELY did.....stilll...). And besides, Gaara no longer has the Shukaku spirit inside him, so his power has been limited. We haven't seen him since that early story arc, but I was under the impression that the sand automatically shielding himn and whatnot was a result of the Shukaku spirit......ah, just my opinions, anyway.
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For me though, while Gaara has the most scariest moves I've seen (ex. sand coffin), Sasuke will be able to beat Gaara because Sasuke has the Sharingan. He will learn Gaara's moves as the battle progresses.[/b]
Yeah, except that Gaara's jutsu involves the manipulation of sand. As Sasuke doesn't have any control over sand, copying Gaara's moves would be useless. *learned this the hard way via Ultimate Ninja 2*