Combine the sugar, water, and 2 rosemary sprigs in a small saucepan over medium heat. Stir until the sugar fully dissolves, then remove from heat and let the syrup cool for 10 minutes.
Remove the rosemary sprigs from the syrup and pour the cooled syrup into a large pitcher.
Add the gin and cranberry juice to the pitcher. Stir well to blend the liquids.
Just before serving, pour in the sparkling water and stir gently to keep the fizz.
Fill four glasses with ice. Divide the pitcher mixture among the glasses.
Garnish each glass with a few fresh cranberries and a rosemary sprig.