When you did your pre-order you needed to pick up one of these boxes off the shelf. The Gold Gun code is inside the box, the flip-side to the card has a scratch off that hides the code.
I think you need to go back to Best Buy and grab one. Because like you said this promotion ends before it's released. I also guarantee you that they won't have any when the game is released, and I don't think they'll have any to give out for those that didn't grab a box. I'm seeing it now, Best Buy employee " Well you needed to grab one of these boxes upon pre-order, so unfortunately I can't help you because they're all gone now".
I was over at Best Buy a few days ago and they had no more of these boxes where I found mine a few weeks ago. If you also look at the bottom of the sticker on the box it says While supplies last. So no Guarantee's you'll get one through Best Buy.