link to returns policy: http://www.gamestop.com/gs/help/Returns.aspx looking at it if you have pre-ordered and game and its still before the release date i would expect you can cancel it, but i dont know for sure.. best to email them i guess, unless someone has experience with it or finds some more info.
http://lmgtfy.com/?q=gamestop+return+policy As far as my extensive research can tell, you'll likely just be given the ~$5 back in store credit.