It seems that GameStop plans on offering AT&T data plans for mobile devices under a service called GameStop Mobile, according to information posted on their website. The information has since been pulled, but not before our friends at Engadget reported on it.
So, as long as GameStop Mobile continues to not be a reality, we will continue reporting the information we've yet to learn about the service. Hypothetically-speaking, GameStop Mobile would require AT&T-compatible or unlocked devices. The service would probably include pay-as-you-go five dollar plans and $55 monthly plans, which would include unlimited voice and text and 500MB of data, or 1GB of data for data-only device users.
The someday-news of GameStop Mobile would make more sense out of the company's recent addition of Android devices to their trade-in lists.