The Gadget plugin module is available only for OSGi-based plugins in JIRA 4.0 and later.
Atlassian gadgets provide the portlets that people can add to their JIRA dashboard in JIRA 4 onwards. To build a gadget, you need a gadget plugin module. The gadget module type can also be included in other Atlassian products.
See the Atlassian Gadgets documentation for details.