How to remove credit card on iTunes so I can use my gift card?

You shouldn't have to remove the credit card, how items are paid for is automatic and iTunes should use the account's balance in preference to the card. If you want to see if you can remove the card : Change or remove your Apple ID payment information - Apple Support

You are doing it just by logging into the account, not when trying to download something ? If you are you could try contacting iTunes Support and see if they can help : http://www.apple.com/support/itunes/ww

You've tried doing it on your computer's iTunes as well as on your iPad ?