I believe Tsunade would win, since she as incredible strength and probably able to beat through his defence and she has incredible healing skills to put her in an almost immortal state for the whole match.
Gaara might be able to win this. He's a long range fighter who's seldomly brought into close combat.
But there really is a lot that can be debated:
-Tsunade's Genesis of Rebirth could regenerate parts of her body being crushed by sand (unless it's her whole body at once)
-Could Gaara's sand even crush Tsunade? Her brute strength might just break free.
-Does Tsunade have the speed needed to rush Gaara?
-Could Gaara's armor of sand withstand a blow from Tsunade? Probably not, but what if he covers himself with diamond-hard minerals and rocks like he used against Kimimaro?
-Where are they fighting? A beach? A rocky mountain?
-Goes Gaara have access to the environment's sand? Or does he only have his gourd sand?
-The list goes on.
You should really try to consider every possibility before deciding a winner. Every factor alters the state of the battle
Let's see Tsunade has power but works mainly in close range atttacks. Gaara has long range abilities and can block Tsunade's attacks, if his sand is strong enough and Gaara has some close range aces up his sleeve.
If Gaara goes airborne, there is no way for Tsunade to land a hit. The only attack she could do is throw up chunks of the earth at him while Gaara rains sand on her.