Grease 8- or 9-inch square pan. Remove any unpopped kernels from popcorn. In large greased bowl, mix popcorn, gumdrops and peanuts.
In 4-cup microwavable measuring cup, microwave marshmallows and butter uncovered on High 1 1/2 to 2 minutes or until melted and smooth, stirring once halfway through microwave time. Pour marshmallow mixture over popcorn mixture; toss until evenly coated.
With buttered hands, press mixture in pan. Refrigerate until firm. For bars, cut into 4 rows by 4 rows.