I followed the instructions on http://simonics.com/2012/03/setting-up-a-linksys-pap2-with-the-google-voice-gateway/ on my PAP2v2 (firmware 1.00.16) but it was not working. The main piece of missing info was the registrar address which is the same as server. Here are all the required entries under Line 1:
In my case SERVER was gvgw8.simonics.com.
Here's what my Registration Status looked like (replaced my number with #):
Registration Status : 1 GV########### Done
Call1 Status : idle
Call2 Status : idle
This allowed me to SEND and RECEIVE calls from my phone connected to the PAP2v2 device. All that was needed was Google Voice + Simonics.
For some reason I see others configuring things like IPKALL + SIP Sorcercy to get calls but I did not need any of that to receive calls.