Update: The instructions here are no longer valid, because Google has since closed off inbound SIP calls into Google Voice. However, I have posted a new solution for making free phone calls with Google Voice, Gizmo, and Asterisk.
I recently switched my GrandCentral account over to Google Voice. I fooled around with Google Voice quite a bit and was quite impressed by the many features. It took quite a bit of work, but I finally figured out how to set up Asterisk along with Gizmo to use Google Voice to make free telephone calls to anywhere in the USA.
Last month, Nerdvittles wrote up a great tutorial on setting up Asterisk to make calls with Google Voice. Unfortunately, shortly thereafter Google Voice blocked inbound SIP calls except those coming from Gizmo. However, it is still possible to set up Asterisk to dial Google Voice if you set up Gizmo to forward calls to Google Voice.