This article will detail for you how to spend a gift card on a purchase.
First, make a sale. When the Process Payment screen appears, select the Gift Card reference field. You can either scan your gift card to populate this number into the field or enter it manually. The amount will be added automatically. You can also enter the amount to charge against the gift card, hit the Enter key which will prompt you to the Reference field to enter the gift card number.
After you hit the Enter key again, you will see the green Complete Transaction button turn green which will allow you to complete the sale while referencing the correct gift card to have funds subtracted from.