I have mixed feelings about Gamestop. It's kind of a love/hate relationship I guess.
But I don't think that GS is in any danger of going out of business any time soon. The company makes money hand over fist selling used games. Further down the road I think they may find trouble unless they change their business plan though. Right now physical media is still the bread and butter of the game industry, but I think that will unfortunately change over the next decade. As physical games become less common GS will have to shift gears to survive and thrive. I imagine them either slowly dying out, or eventually going back to dealing retro games again.
Either way, I don't think that Walmart's decision to enter the used games business is going to make much of an impact at all. Both Target and Best Buy deal with used games already and it probably hasn't even made a dent in Gamestop's profits.