How to pay with my Apple Gift Card balance in Apple Pay without a Credit Card

I got an Apple gift card with $100, and deposited it into my app le account. That worked out fine, but I'm trying to buy something from an online store that takes apple Pay. It keeps asking me for credit card details, how can I pay using the balance from my account? I don't have a card to use for details, can gift cards not buy you anything other than apple products??

[Re-Titled by Moderator]