To make the simple syrup, add the sugar and water into a small pot and warm over medium heat until all the sugar is dissolved. Add in the rosemary sprigs and let it simmer on medium/low heat for five minutes. Turn the heat off and allow the mixture to sit for another 10 minutes before straining out the rosemary.
For the drink, squeeze some fresh grapefruits and add 2 oz of juice to a cocktail glass with ice. Add 1 oz gin and 2 tablespoons of the rosemary syrup to the glass and stir.
Top off with seltzer water. To make it look a little fancy, you can add a rosemary sprig and use a vegetable peeler to peel some of the grapefruit rind for a garnish.