It's listed as a mystery coupon but everyone has reported that it's always a coupon for "25% Off Any Pre-Owned Purchase" that expires on 11/17.
If it ever changes into something else I'll update this post.
25% Off Any Pre-Owned Purchase Subject to availability by location. Limit 1 coupon per customer. Coupon must be surrendered upon redemption. Prices subject to change. While supplies last. 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 Valid 10/28/13 through 11/17/13.