I think the fight could go either way to be honest. Tsunade is extremely powerful and it wouldn't take much for her to kill Gaara if she was able to actually break his defense. But I think your making it sound easier to accomplish this than it actually is. Not only is Gaara a long ranged fighter, he's also very tactful and can do a fantastic job at keeping the enemy away through his long ranged attacks.
Gaara has great reflexes as shown in his fight against Rock Lee, he was able to keep up enough with Lee's incredible speed to actually avoid his attacks(Until he actually used the gates). I'm not saying Tsunade is slow, but she's certainly not as fast as Rock Lee. This will make it even tougher for Tsunade, because as far as we know her only means of attack is physical.
Gaara has also been shown to be able to use the body flicker technique, which can allow him to close the gap against Tsunade very quickly, or if needed escape her momentarily to reform a defense against her next attack or throw an attack out himself. It's really hard to say who wins, but I think Gaara has a slight edge just based off of his fighting style. No disrespect to Tsunade here.