Is it possible to use a Google Play Gift Card with In-App billing and purchases? I have installed the free version of an app on my phone from Google Play. Now I want to purchase the full version of that application using in-app billing or purchase option already available in that application.

I don't have a credit card, but I have already purchased Google Play gift cards, and I want to pay for the in-app purchase with a gift card.

Is this possible, and if so, how?
It is possible 

just click through the in app purchase dialog and there will be a "redeem" button. if you click it, you can simply enter your code and that's it.