Peel grapefruit, separate sections and remove membranes. Place sections in a bowl overnight.
Cook sugar, juice, cornstarch and salt until thick and clear. Add gelatin and stir until dissolved.
Brush some of the gelatin mixture over pie crust. Chill piecrust and remaining gelatin mixture.
When gelatin starts to thicken, add grapefruit sections. Pour into pie crust and chill until set. Top with whipped cream.