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Personally I wouldn't choose either one to win, the out come would be a tough one to call and here's why. Also we're not being specific saying Gaara. Theres the original Gaara, their's the Gaara after battling Naruto, and then their's Naruto Shippuudens Kazekage!

Gaara: Gaara and all his awesomeness only uses sand but can turn earth into sand, but that takes chakra so he only does it if his opponent is tough. Shukaku only comes out when Gaara is Knocked out or goes into a un-natural sleep. AND if Shukaku does come out then it is no longer Gaara fighting since he (in his younger life) had no control over Shukaku. Toph would have a hard time fighting because of his sand use, since she sees with her feet, the sand, as she puts it, makes her vision "kinda fuzzy." Gaara is just not a force to mess with when it comes to his sand coffing and Desert Burial! Sand can blast through metal, and he turns solid rock to sand!

Toph: Toph can bend earth, and she could bend sand as well. Of course as we all know Toph is blind, but sees with her feet, as I stated above, and Gaara is usually covered in his sand armor ( I wonder if she can see him, since the sand armor is kind solid once Gaara uses it.) But Toph can cause great tremors and earthquakes as well as cause major sharp rock formations to suddenly shoot out of the ground from underneath her target. All Toph has to do is make the sand into solid ground to prevent Gaara's attacks!

Both Gaara and Toph have to use different movements to use most of their attacks as well! So all Gaara has to do is move the sand with his will and all Toph has to do is shift her body slightly. To me its pretty much an even match.

toph would KEEL gaara! and then say a sassy remark mrgreen

I suppose it would depend on the terain and which gaara we're talking about.
Pre-timeskip Gaara is bat s**t crazy and prone to using shukaku which would slaughter Toph in Seconds.
Gaara rehab is less crazy and more prone to wipespread attacks which would make for an even fight.
Post time skip gaara has mastered sand which again would make for an interesting battle.
In the desert Gaara would have a massive advantage and would more than likely win, actually I take that back he would win. Toph's ability in the desert is limited due to the fact that she has to manipulte every grain individually this will take much more consentration on her part while for Gaara it would be effortless. Not only that but she has already stated that sand makes it harder for the soundwaves to reach her making it harder for her to pinpoint Gaara's location.
In a forest or any other inviroment, Toph would have a slight to moderate advantage. as the ground is more solid and easier for her to manipulate.
Now abilities wise the match is extremely even.
With all the variables taken in I'd say Gaara has a better chance of victory due to shukaku which trumps anything Toph could pull out of the bag. but again Toph is crafty and you never know what she's capable of.