GameFly is a rental service for games not unlike Netflix. They're 100% legit and actually quite good. They're starting to go into the digital distribution world, and apparently now own D2D. You probably won't see much of a change. In fact, you'll probably have an increase in service level.
OK, easy peasy: They'll make your account a GameFly account. If you already have one, like mine, they'll merge them.