How to Make It
Place grapefruit peel strips and rosemary sprig in a 2-cup glass measuring cup or a sturdy glass; sprinkle with sugar. Muddle mixture with a muddler or wooden spoon until the peels release some liquid, 30 seconds to 1 minute.
Add grapefruit juice and gin; stir until sugar dissolves. Remove and discard the grapefruit peel strips and rosemary sprig. Stir in tonic water. Pour into 2 ice-filled collins glasses; garnish with additional rosemary sprigs, if desired.