Cook sugar and 1 1/2 cups water in a saucepan over medium heat, stirring occasionally, until sugar has dissolved. Bring to a boil. Stir in plums; remove from heat. Refrigerate syrup until cold.
Fill 10 tall glasses with ice. Squeeze 1 lime wedge into each glass; add wedge to glass. Stir 1/4 cup of the plum syrup and 1 ounce gin into each glass. Add sparkling water to fill; stir. Garnish with plums. Serve.