From a post on Apple forum
LINK:
Peeling off the seal on the back of the Apple Store Gift Card reveals a QR code alongside a letter code, like that which appears on iTunes Gift cards.
Open the Passbook app and tap the plus sign in the upper right corner to add the card. You may have to swipe down on existing passes/cards to reveal the plus sign.
Tap on the lower "Passes" panel and aim the camera viewfinder at the QR code, and it will read it. It won't read the letter code.
I don't have a physical card to try out - let me know if it works.
-Craig