According to an advertisement posted on ResetEra, the subscription service, called Power Pass, lets you rent a used game, return it, and then rent another title as often as you'd like over a six-month period. At the end of this six-month period, you'll get to keep any one of the games you rented.
You'll only be able to rent games from GameStop stores and not via the company's online library. You'll also be able to purchase the service for someone else, so as long as they too are a PowerUp Rewards member.
Plans for the Power Play service have since been confirmed to Polygon by a GameStop representative.