You can already set up the Gizmo jabber connectivity as a separate account on top of the GizmoSIP account in the native telepathy (making setting it up in Pidgin redundant). The only problem is that it requires you to edit every contact that shows up in that account in order to add the SIP address (essentially a duplicate of their jabber address) since the Gizmo SIP account doesn't do the presence. The way they have it laid out is not so bad but automatic merging of contacts would be nice.

So while Pidgin is a nice piece of s/w ... it doesn't help here.