How do i use a gift card and NOT the credit card on file

Once the gift card is redeemed onto the account, it will be automatically used to pay for any purchase made. There is nothing that needs to be done beyond redeeming the card: link-> Redeem your App Store & iTunes Gift Card - Apple Support

This does not preclude the requirement of having a verified payment method on the account however. If the payment method had an issue and cannot be charged, it will still be necessary to verify the payment method before the purchase can be completed and charged to the gift card balance.