If you buy books or apps or a single issue magazine etc, the gift card is used .Magazine subscriptions go on the Credit Card.
To check your balance sign in to your account and then go to Gift Cards/Certificates and click on Manage Gift Cards. You can see your balance there. I have 2 gift cards on my account and I can see both balances. When the first card is used up, then the second card will be used. If you don't have enough credit for a purchase on the 1st card, then the second will be used. So if you have a balance of $2 99 and your purchase is $3.01 then you can't use that gift card. Your second one will be used or your credit card.
Remember ... to check balance click on MANAGE GIFT CARDS not Check balance.