In a small saucepan over medium-high heat, bring sugar, rosemary, salt, and ½ c. water to a simmer. Reduce heat to medium-low to maintain a gentle simmer and cook, stirring often to dissolve sugar, about 2 minutes. Let cool slightly.
Transfer syrup and rosemary to an airtight container. Refrigerate until ready to use.
Make Ahead: Syrup can be made 3 days ahead. Keep refrigerated.
In a shallow bowl or plate, mix sugar and paprika.
Gently rub edge of a tall glass with grapefruit wedge, leaving a small amount of juice on rim; reserve wedge for serving. Roll glass in sugar mixture to coat rim.
Fill glass with ice. Add sparkling water, grapefruit juice, and ¾ oz. rosemary syrup. Using a cocktail spoon, stir to combine. Push rosemary sprig through center of reserved grapefruit wedge and garnish glass.
