can you use an apple gift card to pay for your Apple Card balance?

No. You need to pay using your bank.

In order to pay down your apple credit card, the funding source must be couched in terms of a routing number and an account number. So, for example, you can’t use debit cards or credit cards to pay down your apple credit card. Following in this vein, I would say that you cannot use apple gift cards to pay down your Apple credit card.