The aromatic flavor of gin, which comes from juniper berries, complements the bitter-tart taste of grapefruit. Use orange juice as a non-alcoholic option.
In a 3-4 cup pan, combine sugar and 2/3 cup water. Boil, uncovered, over high heat until reduced to 1/2 cup. Gently twist lemon peel and drop into syrup; let col. If made ahead, cover and chill up to 3 days.
With a sharp knife, cut peel and all white membrane from grapefruit. Over a bowl, cut between membranes to release fruit segments; squeeze juice from membrane into bowl. Discard membrane. If made ahead, cover and let stand up to 3 hours.
Spoon fruit, juice, and syrup equally into 6 dessert bowls. Garnish with leaves. To each portion add to taste about 1 tablespoon gin or orange juice, and juice from lemon wedges.