Documentation for JIRA 4.1. Documentation for other versions of JIRA is available too.
The Gadget plugin module is available only for OSGi-based plugins in JIRA 4.0 and later.
Atlassian gadgets are the new way to add portlets to the Dashboard from JIRA version 4 onwards. The gadget module type can also be included in other Atlassian products.
See the Atlassian Gadgets documentation for details.