The advantage gaara has is that his sand protects him automatically. I don't believe the same applies for byakuya. Gaara can also increase the amount of sand under his control by making more sand. I don't know what byakuya would do up against that avalanche of Gaara's.
However, if Byakuya went straight for the kill, his flash step would probably be on par with no-weights lee. In which case, Gaara would be killed in seconds.