Gaara flies at the same general height as his opponents though, along with being obviously one of the most intelligent we've seen thus far. Once he sees the giant Deidara explode, and Deidara hovering above an easy counter to C4 is sand sensing, the sand would undeniably coat the nano bombs, allowing Gaara to sense them.
A grain of sand can be as small as 0.004 mm, so they'd easily do the job.
Why wouldn't you buy it? How is Gaara being caught in Water Dome when he's in the air in the first place? That doesn't make sense to me. Literally he's already in the air, and he sees Kisame spewing out water, that progresses into a dome, what would you do? Sit there, and let it engulf you?
Why doesn't Gaara manipulate his sands gravity? ... Sand isn't supposed to levitate, sand isn't supposed to rise from the ground, and attack his opponents aerial, or on the ground. Gaara controls his sands gravity, there's literally no other explanation for what he does there. Controlling the gravity of oneself doesn't mean you'll repel physical attacks where are you getting that from? That's like saying because Deva can fly with his gravity manipulation, he's immune to physical attack passively. How does that make sense? Deva was hit multiple times by Naruto, yet he's supposed to be "immune"... Nagato was hit multiple times yet he's supposed ot be "immune."