When issuing a gift certificate to someone in the control panel, you have the ability to specify what the gift certificate is valid for. Start by going to Issue New Gift Certificate under the Sell Suite.
Select the person you are issuing/selling this gift certificate to. Enter the amount for the gift certificate.
Once added, scroll to the "What is this valid for?" settings to select what this gift certificate can be used for.
If you want it to apply to multiple items, you can assign it to Items in a Category. This works for any item that supports multiple categories.
When someone is checking out for an item that this gift certificate is valid for (or in the category), it will show up as part of their available balance. In the event they are checking out for something that this gift certificate is not valid for, we will display it as "unavailable".