Fill a glass with ice. Pour the grapefruit juice and gin and stir to combine.
Top with the tonic water and serve immediately.
Notes
You can garnish with fresh herbs. Depending on the season you can use rosemary, thyme, mint or even basil.I love it as is but if you like it with a more complex (and slightly more bitter) flavor profile you can add 3 drops grapefruit bitters to each cocktail.Similarly if you want it sweeter you can add ½ Tbsp honey to each cocktail.