In a large saucepan, bring rhubarb and water to a boil. Reduce heat; simmer, uncovered, for 10-12 minutes or until rhubarb is very tender. Strain and discard pulp. Return rhubarb juice to the saucepan; add the grapefruit juice, sugar and lemon juice. Bring to a boil; cook and stir until sugar is dissolved. Cool slightly. Cover and refrigerate until chilled. Just before serving, transfer to a punch bowl and stir in ginger ale.