possible to use Passbook for gift cards
Yes - this is a valid use of Passbook now called Wallet on iOS 9. 


https://developer.apple.com/wallet/


Apple has ample sample code, working server implementations, automation and testing tools as well as several guides to let you know what is possible without digging into the code.

Most developers that do any sort of work with Passbook could quote you their assistance in running your campaign if you didn't want to DIY.

One major retailer that has had stored value in Passbook for more than a year now (2013) is Apple - both their retail gift cards and iTunes store/Mac App/iOS App/iBooks store combined balances. You can watch a video explaining how Apple rolled this out:


https://developer.apple.com/videos/play/wwdc2013-303/


I also believe that several companies are rolling out discounts/loyalty cards in addition to stored value - but that's a bit newer and I'm not sure that API is finished on Apple's end quite yet. You could easily accomplish that on your end with a web server and some custom coding to track your user through an iOS app or purely through the wallet and/or iBeacons to have the item check in with the server when presented at a POS terminal or other location in store.