How to Make It
In a small saucepan, bring the sugar, 4 of the rosemary sprigs, and ½ cup water to a simmer. Remove from heat and let cool.
Strain the sugar syrup into a large pitcher. Add the grape juice, gin, club soda, and lemon juice and stir to combine. Serve over ice; garnish each glass with grapes and 1 of the remaining rosemary sprigs.