In a small bowl, sprinkle gelatin over 2 tablespoons of the juice; let stand until soft, about 10 minutes. In a small saucepan, bring sugar and 4 tablespoons juice to a boil. Combine remaining tablespoon juice and cornstarch in a small bowl. Stir until dissolved; whisk into boiling orange juice. Remove from heat. Stir in softened gelatin. Cool to lukewarm, and pour over cake.