1. With sharp knife, score peel of each grape fruit into quarters; pull from fruit. (Reserve fruit to use in salad.) Trim off as much white pith from peel as possible; cut peel into long, thin strips. You should have about 4 cups firmly packed peel.
2. In 4-quart saucepan over high heat, heat peel and 8 cups water to boiling; boil 15 minutes. Drain; rinse. With 8 more cups water, boil peel 15 minutes again; drain.
3. In same saucepan over high heat, heat corn syrup, 1 3/4 cups sugar, and 1 1/2 cups water until boiling, stirring frequently until sugar is completely dissolved. Gently stir in peel. Reduce heat to medium-low; cook until most of syrup has been absorbed, about 40 minutes, stirring occasionally.
4. Remove saucepan from heat; gently stir in gelatin until dissolved; cool 10 minutes. (Mixture will be thin and sticky.) On waxed paper, place 1 cup sugar. Lightly roll peel, a few pieces at a time, in sugar; place in single layer on wire racks; let peel dry 12 hours or overnight. Store in tightly covered container.