STIR boiling water into dry gelatin mix in medium bowl at least 2 min. until completely dissolved. Add seltzer, lemon zest and juice; stir. Pour into 9-inch square pan; cover.
FREEZE 3 hours or until frozen. Remove from freezer; let stand at room temperature 10 min. to soften slightly.
BEAT with electric mixer or blend in blender on high speed until smooth. Spoon into six dessert dishes. Store leftovers in freezer.