Apple purchase using iTunes gift card

You need to redeem the iTunes gift card to your account: Redeem App Store & iTunes Gift Cards and content codes - Apple Support

Once redeemed its balance will appear in your account and can be used to purchase items automatically: View your Apple ID balance - Apple Support

Apple will use any gift card balance on your account first before charging any other associated payment: How App Store and iTunes Store purchases are billed - Apple Support

You may still have to verify credit card number on your account if required, but the charge will be taken from the balance first.

Verifying a credit card payment method is one form of the system verifying that it is actually you attempting to use the account. Only you should be able to verify your payment method. In my experience, it is usually looking for you to supply the 3 digit code.

Apple has stated that credits on an account are used before billing a credit card as linked above.

ASC - Late 2012 Mac mini, macOS 10.14.6; Watch, series 5 GPS/cellular, watchOS 6.2; iPad Air 2, ipadOS 13.5.1; iPhone 8+, iOS 13.5.1; TV 4th Gen, tvOS 13.4.6;  HomePods, audioOS 13.4.6 and Airport Express w/ Airplay 2

Did you use the method described in: Redeem App Store & iTunes Gift Cards and content codes - Apple Support?

wajd288 wrote:

If i verify credit card number will it be taken from the credit card?

No. It should be charged from the iTunes gift card balance first.