Cut each grapefruit in half; remove sections, reserving juice. Set sections aside. Add enough water to juice to measure 1-1/2 cups.
In a small bowl, sprinkle gelatin over cold water; let stand for 1 minute.
Meanwhile, in a small saucepan, combine the sugar, salt and grapefruit juice mixture. Cook and stir until sugar is dissolved. Remove from the heat; stir in gelatin until completely dissolved. Stir in vinegar. Chill until slightly thickened, about 1 hour.
Fold in cucumber and grapefruit sections. Transfer to a 1-qt. mold coated with cooking spray. Refrigerate until firm.
Unmold onto a lettuce-lined serving plate if desired.