In a saucepan, combine pineapple and sugar; bring to a boil, stirring occasionally. When mixture boils, immediately add gelatin and stir until dissolved. Cool slightly. Stir in buttermilk. Chill until partially set. Fold in whipped topping and nuts. If necessary, chill until mixture mounds slightly. Pour into a lightly oiled 8-1/2-cup mold. Chill overnight.