Add drained liquid to grapefruit juice; measure.
Add enough water to make 3 1/2 cups.
Soften gelatin in 1 cup of the liquid.
Heat remaining liquid and add softened gelatin; stir until dissolved.
Chill until consistency of unbeaten egg white.
Arrange some of the grapefruit sections and sliced olives in bottom of a 5 cup ring mold.
Add enough of the chilled mixture to cover sections and olive slices.
Chill until almost firm.
Repeat procedure, reserving some of the grapefruit sections and olive slices.
Chill until firm.
Unmold on serving platter.
Surround grapefruit ring with salad greens and garnish with remaining grapefruit sections and olive slices.
If desired, fill center with salad greens, chicken salad, tuna fish salad, or cottage cheese.