How do I ensure in-app purchases are charged to Apple Gift Card instead of Apple Card?

In-app purchases always charge your Apple Account balance first, before there are any charges to your payment method. You would first need to redeem the Apple Gift Card, so the balance shows in your Apple Account balance. They only way your Apple Card would be charged is if there is enough enough money for the purchase in the Apple Account.

May want to provide a screenshot, but you would want to redact your Apple ID. It may be an option to use Apple Pay if you tap on that instead of your Apple Account. By default, in-app purchases will bill your Apple Account first.