That's because GameStop totally revised their downloader and you may have to redownload it fresh off their website and reinstall it to get everything working properly. Also, have you checked that you're properly logged-in? I know it was throwing me errors for a while before I noticed the very tiny "Login Failed" box on the bottom-right of the window. If you have one of those, click on it and input your log-in info and it should work. If it doesn't, re-install from a fresh download and that should work.