You can sell the game back to them for whatever the current trade-in value is, but you can't return it as a new game. Gamestop can not legally re-sell the game as new and the bundle usually states that the individual components are not for individual sale for that very reason.
Try selling it on ebay or craigs list and you may get a better price.
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Ok this is really annoying. You CAN NOT RETURN THE GAME FROM THE BUNDLE!!!!!! You can SELL/TRADE IT FOR STORE CREDIT. YOU CAN NOT TRADE 1 GAME FOR ANOTHER unless the store credit is as much as the game you are buying. Gamestop won't give you much for it. Generally speaking, bundled software and accessories do not fetch very high store credit. The Edge Card (which you have to pay for) might get you an extra dollar at best.
I managed Funcolands for 5 years. Software etc (Gamestop) bought them in the early 2000's. That's where Game Informer and the 'Edge Card', as it's now called, came from. I additionally worked at Software etc for another 4 years... first as a key holder and then part-time while in school. So I know what I'm talking about. Sell it privately if you want a little more money, or trade it to a reseller... but you can't 'return' it!