Before you could only use those Apple Store gift cards you got at the actual site or in physical stores. Now, you can shop even from the Apple Store app for the iPhone and iPad. Yes, the iOS version now supports gift cards. To use a gift card for an order, you just need to tap on the “Add a gift card” option on the Checkout page. You can either manually type the PIN or scan it using the camera of your iOS device.
Source: Cult of Mac | Download: Apple App Store (Free)