Mix pineapple and sugar together in a medium saucepan, and bring to a low boil, stirring occasionally. Stir in gelatin until completely dissolved, remove from heat, and cool to room temperature. Add buttermilk, and refrigerate 1 hour, or until partially set.
Gently fold in whipped topping. Pour mixture into a lightly oiled gelatin mold. Refrigerate 8 hours, or until completely set.
