How do I use gift card to buy more from downloaded games?
How do I use a gift card to purchase more coins for a game app downloaded to my iPad instead of using a credit card?
iPad 2 Wi-Fi + 3G
If you have a balance on your account then that should be automatically used, your credit card should only be charged for any remainder if your balance doesn't cover the price (your credit card will also be used if you decide to gift anything).
From http://support.apple.com/kb/HT5582 :
When making purchases, content credits are used first, followed by Gift Certificate, iTunes Card, or Allowance Account credits; your credit card or PayPal account is then charged for any remaining balance.