The agreement has been rumored, but investment advisor DOMO Capital Management claimed via Twitter that it had received confirmation from GameStop.
Basically it means that the cash strapped chain will get a share of all downstream revenue for customers it brings into the Xbox ecosystem this generation. Essentially, if a customer has purchased her or his Xbox Series X or S from any GameStop branch, the retailer will get a share of each digital purchase the user makes, whether its full-game downloads or downloadable content.
DOMO even claims this applies when the DLC is being purchased for a physical base game that was bought at another retailer, providing the DLC is being bought from the Xbox store. This also extends to pre-owned Xbox Series X and S consoles, with GameStop reporting to Microsoft every unit that it sells. It's unclear whether a similar arrangement exists with Sony.