Combine sugar and water in a small saucepan and bring to a boil. Boil until the sugar dissolves and the syrup is clear. Remove from heat, add 1/2 cup of the mint leaves and steep 15 minutes. Strain syrup, let cool and refrigerate until chilled. (The syrup will keep, refrigerated, for 2 weeks.)

In a bowl, mix the pink grapefruit juice with the syrup. Pour into an ice cream machine and freeze according to the manufacturer's directions. Transfer to a container and freeze until serving time. Wrap and freeze the reserved grapefruit halves. Meanwhile, peel the grapefruit or pomelo and pull it apart into segments. Using the tip of a small sharp knife, loosen the white membrane surrounding each segment and pull it away, trying not to pierce any of the little juice pods. Break up the segments into the juice pods and set aside.

Place the sorbet in the refrigerator for 15 minutes before serving to soften slightly.

Scatter half the grapefruit or pomelo juice pods over the bottom of the frozen grapefruit halves. Scoop sorbet over the grapefruit and cover with remaining grapefruit juice pods. Garnish with mint leaves.