In a Dutch oven, bring rhubarb and water to a boil. Reduce heat; simmer, uncovered, for 10 minutes. Drain, reserving liquid (save rhubarb for another use).
In a large bowl, dissolve sugar and gelatin powder in boiling water. Stir in pineapple and lemon juices. Stir in rhubarb liquid; refrigerate until chilled.
Just before serving, pour into a punch bowl and stir in ginger ale. If desired, garnish with fruit.
