For the rosemary simple syrup: Add the sugar, 1/2 cup water and the rosemary to a small saucepan and bring to a boil, stirring until the sugar is dissolved. Remove from the heat and let cool.
For the champagne fizz: Combine a tablespoon of rosemary simple syrup (more if you like it sweeter), the gin and grapefruit juice and stir. Pour into two cocktail glasses. Top each with champagne and garnish with a sprig of rosemary. Enjoy!