Spray 24 mini muffin pan cups with cooking spray; set aside. Add boiling tonic water carefully to gelatin mixes in large bowl (gelatin will foam); stir 3 min. until completely dissolved. Skim off any residual foam.
Spoon 1 tsp. of gelatin into each mini muffin cup. Place grape piece in the center. Chill 5 min. Spoon remaining gelatin over grapes. Refrigerate 3 hours or until firm.