You can but it's completely separate to the Google Voice mechanism. To receive calls to and from GTalk with sipsorcery you will need to use a 3rd party provider such as ippi that provides a SIP gateway to Google's GTalk network. You can read about how it works on ippi's site here http://www.ippi.com/index.php?page=news&nid=99&lang=44. As far as setting it up with sipsorcery it's the same as using ippi and Skype but change the Skype references to GTalk http://www.sipsorcery.com/mainsite/Help/SkypeIppi.