If you are selling a Gift Card, you will want to first make a new revenue account for gift cards. Also make a new sales category for gift cards, use the Gift Card account as its merchandise account.
When the customer purchases the card you will put in a new Miscellaneous Cash Receipt to the new Gift Card account. This will put a credit into the Gift Card account.
To use the gift card you will need to create the items/specifications that they are purchasing. You will also need to make a new item for the gift card, with a negative amount for the amount they are using of the gift card. Be sure the Sales Category for this item is the Gift Card category. Then invoice the client for all items including the negative item amount for the gift card being used. This will take the credit out of the Gift Card account that you put in when doing the misc. cash receipt.