1. Use a vegetable peeler to remove 3 long strips of peel (just the zest, not the pith) from the grapefruits. Set aside. Squeeze out 2 cups of grapefruit juice.
2. In a saucepan, combine the grapefruit peel, sugar, corn syrup and water. Bring to a boil, stirring to dissolve the sugar, and cook for about 2 minutes. Set aside to cool. Place in the refrigerator, or set in a bowl of ice for faster chilling. Discard the peel.
3. Strain the grapefruit juice through a sieve or strainer to remove the pulp. Discard pulp. Stir the sugar syrup into the grapefruit juice. Pour into the container of an ice cream maker, and freeze according to the manufacturer's instructions. Transfer to a container and freeze until firm before serving.

Ready in 1 hr 35 min