Nintendo accounts for the most new sales at 23%. Following Nintendo are Sony (17%), Microsoft (12%), Electronic Arts (12%) and Activision (11%).
The next five companies on the list account for another 10%, leaving the remaining 65 companies fighting over the last 15%. If anything were to happen to any of the top 5, GameStop (and likely all game retailers) would take a large hit.
GameStop also has a lucrative used hardware and software business.