GNOME is made of many source packages: one should download the gnome-libs (core GNOME libraries), gnome-core (core GNOME desktop) and bonobo (obviously, bonobo stuff) packages. Here is a list of the idl files which are included in there.
gnome-unknown.idl: this file can be found in gnome-libs/idl. It details the GNOME:Unknown interface.
gnome-factory.idl: this file can be found in gnome-libs/idl. It details the GNOME::GenericFactory interface.
name-service.idl: this file can be found in gnome-libs/idl. It details the various interfaces implemented and used by the GOAD
Table.idl: this file can be found in gnome-libs/idl. It details a GNOME::Table interface... I wonder what it could be used for ...
desktop-editor.idl: this file can be found in gnome-libs/idl. It details various interfaces which must be implemented by a text-editor to integrate well in a GNOME environment.
desktop-textviewer.idl: this file can be found in gnome-libs/idl. It details various interfaces which must be implemented by a text-viewer to integrate well in a GNOME environment.
gnome-panel.idl: this file can be found in gnome-core/idl. It simply details the various interfaces implemented by a good GNOME Panel and its applets.
help-browser.idl: this file can be found in gnome-core/idl. It simply details the various interfaces implemented by a good GNOME help browser.
control-center.idl: this file can be found in control-center/idl. It simply details the various interfaces implemented by a good GNOME Control Center and its applets.
gnome-container.idl: this file can be found in bonobo/idl. Of course, we have there the GNOME::Container interface.
gnome-embeddable.idl: this file can be found in bonobo/idl. Very surprisingly, the GNOME::Embeddable interface is detailed there.
gnome-client-site.idl: this file can be found in bonobo/idl. The Container GNOME::ClientSite interface definition.
gnome-ui-handler.idl: this file can be found in bonobo/idl. Do you want in place activations ? Just implement this set of interfaces.
gnome-storage.idl, gnome-persist.idl and gnome-progressive.idl: these file can be found in bonobo/idl. They are used to implement saving support on components.
gnome-advise.idl: this file can be found in bonobo/idl. A copy of the IAdviseSink interface from COM/DCOM. Not used yet. Is it deprecated ?
gnome-moniker.idl: this file can be found in bonobo/idl. Implement linking support on your components !
gnome-canvas.idl: this file can be found in bonobo/idl. Experimental: Non square components !!
gnome-control.idl: this file can be found in bonobo/idl. Experimental: Controls like with activeX.