Spray large cookie sheet with cooking spray. In 4-quart saucepan, melt butter over low heat. Add marshmallows; cook, stirring constantly, until completely melted.
Stir in snack mix and red and green gumdrops until well mixed; pour onto cookie sheet.
Spray sheet of waxed paper with cooking spray. With paper, sprayed side down, and hands, shape mixture into wreath shape with 4-inch hole in center. Cool completely, about 45 minutes. To serve, cut into slices.