In a cocktail shaker filled with ice, add the grapefruit juice, gin, rosemary simple syrup, and shake vigorously. Pour into a chilled martini glass. Top with champagne or sparkling wine. Garnish with a sprig of fresh rosemary.
In a small pot over medium heat, add the sugar and water. Stir until the sugar is dissolved. Bring to a boil. Once the mixture is boiling, reduce the heat and allow to simmer for 3-4 minutes until starting to thicken. Remove from heat and add the rosemary. All the syrup to cool completely. Remove the rosemary sprigs and store in a covered container in the refrigerator for up to 1 week.