In a large bowl whisk together grapefruit juice, sugar, and gin until sugar dissolves.
Stir in lime and orange zest.
Add lime juice, lemon juice, and salt incrementally to taste. The sorbet should taste as tart as it is sweet. Chill well if grapefruits were not chilled already.
Churn in an ice cream maker according to manufacturer's instructions, then chill for two to three hours in the freezer before serving.
To serve individually, serve scoops in bowls or champagne flutes and top with two to three ounces champagne and about 1 teaspoon pomegranate molasses. Or serve sorbet in a large mass in a punch bowl, scooping off chunks as needed.