Drain the grapefruit sections and add enough grapefruit juice to the syrup to make 3 1/2 cups.
Put 1 cup of the grapefruit juice in a small saucepan, sprinkle the gelatin on it, and heat over low heat until the gelatin is dissolved, stirring constantly.
Add the sugar and salt to the hot juice and stir into the rest of the juice.
Chill until the consistency of raw egg whites.
Fold in the well-drained grapefruit sections, the avocado, and the celery.
Pour into a mold rinsed out in cold water, and chill until set.