I pre-ordered RDR and ended up receiving two Golden Guns codes because of it (one in the box and one e-mailed to me from Amazon).
 
I assume they both work and since I only need one I thought I'd throw the other one up on the boards for anyone who's interested.
 
This is for the PS3 version of RDR in the US:  5YHB-ZL3E-DZBE-X65E

Same thing happened to me.  I still haven't received my gift card though.  Have you?

I get the impression this happened to all of us who pre-ordered from Amazon. The golden guns are pretty much useless to anyone who's started the game already, I should think… 
@Kahoona:   You won't be receiving a "gift card." The credit is automatically applied to your next game purchase.

" @Kahoona: You won't be receiving a "gift card." The credit is automatically applied to your next game purchase. "

I used the code so i would recommend no one else try to enter it...sorry :)

It did say a $20 gift card, you're right, it's applied to your Amazon account directly.  I haven't seen one show up on mine yet, but I think it can take up to 10 business days if I remember from my experience with the Heavy Rain pre-order bonus.

Well my friend ordered RDR on the Tuesday and he got his email about the gift card the same day.  I pre-ordered two months ago and only got an email for another golden gun code.  Perhaps it is an error on their part where they sent pre-orderers an extra golden gun code instead of a gift card code?