If you've been following the Play Store-related news cycle recently, you are probably well aware that Google is on the verge of releasing gift cards for use on Play Store's digital goods. The Redeem functionality, along with a new Wishlist, came with the new Play Store APK v3.8.15 that started rolling out yesterday but, unfortunately, the UI does not expose it yet.
There is no web interface for redemption either, but after digging through the APK, I discovered that it was set to intercept calls to http://play.google.com/redeem. So, while you can't pop up the Redeem Activity in the app directly without hacking, removing the stock app, and installing the re-signed hacked version, there is a much easier way. I just successfully redeemed a gift card, and so can you.
Here it is:
Step 1. Install the Google Play 3.8.15 APK by downloading it to your device and installing as a regular app.
Step 2. Get the http://play.google.com/redeem link to open up in the Play Store instead of your browser. One easy way to do that is to email yourself the link and select Play Store while opening it, like so:
Step 3. You're done - say hello to the Redeem page.
Step 4. Enter the code and press Redeem.
Step 5. Now spend the Play Store balance on digital goodies and make some developers happy.
And there you have it - a way to trigger gift card code redemption in the Play Store ahead of Google's official rollout.
P.S. Sorry, UK and likely rest of the world outside the U.S., Play Store gift cards are likely not going to be in your market at launch. Here's the message UK customers get on the Redeem page right now (thanks, Jack Fifield!):