how to put a apple gift card on apple pay

Unfortunately you can’t do that.

However you CAN redeem your gift card to your AppleID and from there use it to purchase Apple goods and services.

How to redeem your Apple Gift Card or App Store & iTunes gift card - Apple Support

What you can buy with your Apple Gift Card or Apple Account balance - Apple Support

I’m trying to add a gift card to my account

Can u use an apple gift card in apple pay