In the context of GateIn Portal, gadgets are defined by the Google OpenSocial specifications. Since EPP (now JPP) 5.2 and GateIn 3.1, the portal framework includes Apache Shindig 2.0 which is made to support the version 0.9 and 1.0 of OpenSocial.
Within a portal, it is possible to embed any OpenSocial gadget in a page or in a user's dashboard. Gadgets can be added to the application registry and links can be added to the mini-composer - see GUG-Portlets and Gadgets Administration of User Guide.
OpenSocial gadgets are made of standard HTML and javascript. The container offers an API, the documentation for which is available here.
Note that, unlike portlets, a gadget has very little knowledge of its context (the portal) and its integration within the portal may be more limited, for instance in terms of visual integration.
While Google Web Toolkit (GWT) applications can also technically be used as gadgets, we do not recommend its usage. This is because Google's strategy to keep GWT applications running as gadgets cannot be clearly ascertained and consequently, GWT applications are not covered by the JPP Support Agreement.