How do i put my Apple Gift Card balance into my wallet?

Seems a gift card balance can only be used for Apple purchases:

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

Some purchases might require that you have a credit card on file, even if you use your Apple Account balance to make the purchase.

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

Thank you for letting us know we helped, Richard Thomas.

Cheers :-)