1. Strain the juice through a sieve into a bowl, pressing to extract all the juice; discard pulp. Pour half the juice into a shallow bowl; sprinkle the gelatin on top and let stand 5 minutes. Combine the remaining juice with honey in a medium saucepan and heat to a simmer, stirring until the honey has dissolved. Remove from heat.
2. With rubber spatula, scrape gelatin mixture into warm juice and stir until gelatin has completely dissolved (mixture should not feel grainy when rubbed between fingers).
3. With sharp knife, cut peel and pith from grapefruits and oranges; cut between membranes to release segments. Arrange all fruit in 6 (10-oz) glasses and fill with juice mixture. Cover and refrigerate 3 hours until set (or up to 3 days).