Stir boiling water into gelatin in large bowl at least 2 minutes until completely dissolved. Stir in cold diet ginger ale. Refrigerate about 1-1/2 hours or until thickened (spoon drawn through leaves definite impression).
Stir in grapes and oranges.
Pour into 6-cup mold sprayed with cooking spray.
Refrigerate 4 hours or until firm. Unmold. Store leftover gelatin mold in refrigerator.