That's when Gaara had first learned to use desert suspension, granting him the advantage aerial wise. The only reason this may turn into a high-diff is because of Katsuyu.
Gaara beats Tsunade low diff without Katsuyu. High with her. Literally: Sea of Sand -> Sand Burial combination = Dead Tsunade. That, or he buries Tsunade 300m under the ground, and kills her. Kimimaro only survived due to bone forest. Tsunade isn't.