1 cup sugar
1 cup water
1/3 cup packed fresh rosemary leaves
1/3 cup coarsely chopped fresh ginger
1/4 teaspoon black pepper
1 cup gin
1 cup rosemary-ginger simple syrup
1 cup sparkling water
1/2 cup fresh lime juice
10 ice cubes
Bring all syrup ingredients to a boil in a small saucepan over medium heat. Boil, stirring until sugar has dissolve, about 1 minute. Remove from heat. Let stand, covered, 1 hour at room temperature.
Strain through a fine-mesh sieve, pushing down on solids to extract as much liquid as possible. Refrigerate until ready to use.
Combine gin, 1 cup of the rosemary-ginger simple syrup, sparkling water, and lime juice in a cocktail shaker. Add ice cubes and shake until well-chilled. Strain 3/4 cup each into 8 glasses and serve.