I believe Gaara is stronger overall, but this is a bad matchup for him. Gaara doesn't do well with wet sand or chakra draining effects, so the giant sphere of water would be a nightmare for him. Kisame wins this one.
Kisame will win this. Sand is weak against water as seen when 2nd mizu broke the sand bind. Also kisame is strong against ninjutsu dependent shinobis as he will just absorb chakra based attacks.