I want to make an in-app purchase, but it fails and tells me to contact the Itunes support. I did that and they pointed me to

update my billing / payment info. However, I have NO credit card available and payment info is set to NONE. I want to use my gift card for the in-app purchase.

Are in-app purchases only possible with a credit card? That would be weird, because i can usually buy and download apps from the appstore.