HEAT oven to 375ºF. Mix dressing and garlic. Pour over chicken in large bowl; toss to coat chicken. Refrigerate 30 min. to marinate. Meanwhile, pierce squash with fork. Microwave on HIGH 5 min. Cut squash in half; remove seeds. Cut each half into 6 pieces.
PLACE chicken in 2 (13x9-inch) baking dishes; top with squash, butter and sugar.
BAKE 1 hour or until squash is tender and chicken is done (165ºF).