Declined payment method

What kind of gift cards? if they are something like a Visa gift card, that does not appear to be accepted by Apple.

Payment methods that you can use with your Apple ID - Apple Support

You would have to transfer the value to payment method that is.

If you mean an Apple gift card, that has to be redeemed to your account first.

The article "What type of gift card do I have?" explains the different kinds of gift cards Apple uses and has links on how to redeem them and what can be purchased with each kind. Click on the link --> What type of gift card do I have? - Apple Support

Read this support article to learn how to redeem your Apple Gift Card or App Store & iTunes gift card --> How to redeem your Apple Gift Card or App Store & iTunes gift card - Apple Support

Very similar to what is in this document:

If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card read this document about things to try --> If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support

May be its out of balance or insufficient amount that the product your purchasing is of a higher value but before this gift card needs to be redeemed to the the apple account first, check it and try again.