As far as I can tell, you have to have separate connections for the voice and chat sections of the service, and you lose the built-in money meter and call record. This is because Gizmo5 uses Jabber for text messages and SIP for voice, while Google uses their own voice extension to Jabber, called Jingle.

I connect to the text chat with the Jabber client, with <username>@chat.gizmoproject.com connecting to chat.gizmoproject.com

I connect to voice with the SIP client, with <number>@proxy01.gizmoproject.com