In a medium-sized saucepan over medium heat, combine the water and granulated sugar. Stir to combine.
Stirring occasionally, heat the mixture until the sugar has dissolved entirely. Once it has dissolved, remove it from the heat.
Add the rosemary to the simple syrup. Allow it to cool completely. Remove the rosemary and stash the simple syrup in the fridge.
In a cocktail shaker over ice, add the fresh grapefruit juice, gin, and rosemary simple syrup. Shake until very cold and strain into a rocks glass over ice.
Garnish with a quarter moon of fresh grapefruit and a sprig of fresh rosemary.