this is an interesting matchup. both of them when they have their respective element present are rather strong. gaara with sand and suigetsu with water. However, gaara wins easy unless they are next to a large body of water. Suigetsu can utilize large bodies of water effectively as shown when fighting bee.
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and as we have seen from all of gaara's battles, if he has a large amount of sand he is incredibly strong. Now the reason I said gaara wins easy unless near water, is because he can easily make sand.
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So regardless of where they are, gaara can make sand to use. But unless suigetsu is near water, he can't do much. We are yet to see suigetsu able to use a large water jutsu able to produce the water himself.