Make the sage simple syrup: In a small saucepan over medium heat, combine the water, sugar, and chopped sage leaves. Bring to a low boil, then reduce the heat to low and simmer for 5 minutes, until the sugar is fully dissolved and the sage is fragrant. Remove from the heat and let cool for 5-10 minutes. Strain through a fine-mesh sieve. Store remaining sage syrup in the refrigerator for up to 1 month.
In a cocktail shaker filled halfway with ice, combine the gin, grapefruit juice, and 1 tablespoon of sage simple syrup. Shake vigorously for 30 seconds. Strain into a coupe glass.