This will be interesting to watch unfold. We had heard from a source at GameStop that the company will be the exclusive retailer for Steam gift cards, which are reportedly going to show up as early as next week. According to Kotaku, May 15 is the date; the site has also received a photo of a $50 voucher.
I can only imagine what the conversations between GameStop and Valve were like in reaching an agreement, assuming all of this stuff isn't an elaborate hoax. For as popular as Steam is, there are many gamers who do not use the platform. And, presumably, a large segment of them shop at GameStop. Beyond that, another way to use up trade-in credit sounds like a good idea to me.