In order to use gift cards, first you need to create a new "part" category only for gift cards.
- Go to "Inventory management" / "Product/Job code management",
- Click on the spyglass next to category,
- Click on the "+" sign and create new category,
- When you create it check "Inventory follow up" box and "Block list price inferior to the cost" box,
- Leave unchecked "Allow product purchases" because you won't need to purchase the items in this category.
- Go to "Inventory management" / "Product/Job code management" and click on the "+" sing to create a new product,
- When you sell a gift card to somebody the qty field should be positive,
- When you apply a gift card to customer's invoice the qty field should be negative.