A bit late, but thanks anyway.
I didn't get to use my gift card. I called Apple (1-800-My Apple) and talked to an iPhone sales rep. She looked and tried, sent IMs to other staff. The consensus was, no you can't redeem the gift card after purchase.
Well, that's not 100% true.
Let's be more specific. You can't redeem the gift card after purchase, without returning the iPhone. You have to cancel your order, and then order again, this time using your gift card as part of the process.
You cannot receive your iPhone, take it into the store, return it and repurchase. Once you return the iPhone, (if unopened) it goes back in stock, and then gets re-assigned to the next "person in line" YOU have to go to the back of the line and start over. This is the nature of the highly constrained product launch. If the phone is opened, again they can't resell it back to you. It gets sent back to Apple, to the "refurbished" inventory and waits for that process to begin.
So, for me, I have a $100 gift card I can't use towards the purchase of my phone. I do have it though for anything else.