Hey, the BF had to return a defective game just the other day...
If there are any other Gamestops nearby, you could have them check that store for a copy of your game, and hold it till you can get over there to exchange.
I would imagine that, if no one around has it, and there's no other games you want, you'd probably get a refund- but it may be store credit.
(And if they try to swap you discs for a used copy, I'd demand a price adjustment. Just because.)