Sprinkle the gelatin over the cold grapefruit juice, add sugar, if desired, then heat it to a simmer and stir well to make sure the gelatin is dissolved.

Pour enough of the hot liquid into a 3- or 4-cup mold to cover the bottom 1/2 inch. Chill until almost firm.

Peel the avocado and cut into the same number of slices as you have grapefruit segments. Place alternating pieces of grapefruit and avocado on the slightly firm jelly, chill again, and then pour the last of the juice over. Refrigerate until firm. Unmold to serve.