Make cranberry simple syrup: Add granulated sugar, water, and fresh cranberries to a small saucepan over medium heat
Stir occasionally until sugar dissolves and cranberries soften
Gently mash to expel the cranberry flavor, then simmer for 10 minutes
Remove from heat and allow to cool completely
Strain into an airtight container and store in the refrigerator
For serving right away: Add gin, lemon juice, cranberry syrup, and sparkling wine to a pitcher or punch bowl
Add ice and stir to combine
Serve in flute glasses and garnish with fresh cranberries and rosemary sprigs
For making ahead: Add gin, lemon juice, and cranberry syrup to a pitcher or punch bowl and refrigerate
When ready to serve, add sparkling wine and stir, then serve in flute glasses
Alternatively, pour gin/lemon/cranberry mixture into flute glasses half full and top with sparkling wine
