To make rosemary syrup, combine water, sugar and rosemary in a medium saucepan and bring to a simmer, stirring until sugar dissolves.  Remove from heat and let rosemary infuse for 30 minutes, then strain into an airtight jar.

For each serving, fill a glass with ice then add grapefruit juice, gin and rosemary syrup, stir, and then top off with sparkling water. Garnish with fresh rosemary and a small wedge of grapefruit. Cheers!