Cut a grapefruit into quarters and add one of the quarters to an open cocktail shaker. Add in the pomegranate seeds and muddle grapefruit and pomegranate in order to release the juices and oils. You can use the back of a wooden spoon for this step if you don't have a muddler tool.
Add ice and gin to cocktail shaker and securely place the top on it. Shake vigorously for about 10 seconds to fully chill and combine the mixture.
Strain mixture into a coupe glass and top up with chilled tonic water. Garnish with a wedge of grapefruit and pomegranate sides, if desired. Enjoy responsibly!