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 field to enter the Amount. After prompting the amount to charge against the gift card, hit the Enter key which will prompt you to the Reference field. In this field, you will enter the Serial number in which the gift card was issued under. You can either scan your gift card to populate this number into the field or enter it manually.
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.