Go to www.gamestop.com, click on the help tab on the top right, click on the "contact us" link at the bottom of the page, and viola!
Here's the number: 1-817-424-2200.
But seriously, as mietha said... you seriously expect customer service from them? Every time I call, I'm put on hold for at least 20 minutes before I lose the patience and hang up. It's like they have one person working on their customer service. Email them first before wasting up to an hour of your life on the phone.
I'm never ordering from GS online again... I have yet to receive my DDS2 soundtrack which they didn't include with my package. I'm so damn pissed because of that.