To sell gift cards on your Ecommerce you need to enable them from the Ecommerce control panel. Mouse over sell online and select gift cards from the drop down menu.
Select the preferences tab from the top of this section. From here you can click the radio button next to yes under enabled to activate the ability to use gift cards on your website.
Note: In addition to enabling gift cards from the Ecommerce control panel, you must create a gift cards page from the site builder in order for gift cards to be available for client purchase.