To follow up on the situation, I just wanted to say "thank you" to all those who set the record straight in that this (generally) was not a Gamestop issue (other than the store not wanting to help in any way, shape, or form). I've never preordered a game before, and didn't realize the developers tend to not do as they should. Thanks again to those who suggest I simply contact customer service (something again I didn't realize Gamestop had).
Luckily, this story does indeed have a happy ending. I contacted Gamestop's customer support phone number, indicated I had a store issue, and explained what happened. The rep verified my receipt information and said they'd be arranging to get me a preorder code sent to my e-mail address within 48 hours.
Sure enough, almost exactly 48 hours later (on a Saturday no less), I received an e-mail from Gamestop with my Audi redemption code.
I am a happy racer again. :)