In a small bowl, sprinkle gelatin over 1/4 cup water; set aside. In a large saucepan, bring cranberries and remaining water to a boil. Reduce heat; cook until berries pop. Remove from the heat; press cranberries through a sieve or food mill, reserving juice. Discard pulp.
In a large bowl, combine the warm juice, gelatin and sugar until dissolved. Cool to room temperature. Stir in ginger ale.
Pour into a freezer-proof bowl; cover and place in the freezer until mixture is partially frozen. Beat with an electric mixer until light in color. Cover and freeze overnight.