Yes, you can of course do this. You must use the number on your gift voucher and the web code, which is on the receipt you received in connection with the purchase of your gift voucher/receipt of your gift voucher. You can redeem your gift voucher in step two of the payment process.
NB: The number of your gift voucher is under the barcode on the back of the gift voucher. This number is NOT the same as your gift voucher code.