Assuming you are using SIP1/ITSPA for your SIP provider and want to add Google Voice on SP2, set all options to default on:
Service Providers -> ITSP Profile B -> (General, SIP, and RTP)
Voice Services -> SP2 Service
Then change only the following:
Service Providers -> ITSP Profile B -> General -> Signalling Protocol : Google Voice
Voice Services -> SP2 Service -> X_ServProvProfile : B
Voice Services -> SP2 Service -> AuthUserName : [your Google Voice Username]
Voice Services -> SP2 Service -> AuthPassword : [your Google Voice Password]