Using Play gift card to purchase donuts?
You should be able to select the play giftcard. Mine always defaults to my google play balance first. If it doesn't and chooses the CC, just cancel.
Forum Discussion
In my experience (I have a gift card on my account and a credit card too) it will automatically deduct from your Google Play balance (where redeemed gift card amounts go) unless there is less money left in the balance then the app/in-app purchase you are trying to buy.
