Just got an email advertising this. Here's a recent thread that gave out the same coupon by playing a different game on Kongregate.
Small print details:
*Coupon: Limit 1 coupon per customer. Offer valid in stores only. Coupon must be surrendered upon redemption. Coupon not valid for items priced less than $3.00. Cannot be combined with any other offer. See store associate for details. No dealers. Offer valid in the United States, Puerto Rico and Guam only. Void where prohibited. GameStop, Inc. reserves the right to cancel, terminate, modify or suspend the offer for any reason without notice. Offer expires 5/11/14.