First a definition. Forking is routing an inbound call to 2 or more destinations.
You fork inbound GV calls to your OBi202 phone port and NoMoRobo. If the callerid is in NoMoRobo's database, they will block the call by answering it, otherwise they will produce a Busy Signal and the OBi202 will continue to ring. You can open an account on NoMoRobo, but that's not necessary. I think they assign the same 800 number to everybody.
You need to define a dummy trunk on the OBi202 then fork calls to NoMoRobo using a free tollfree provider.
Service Providers -> ITSP Profile D -> SIP -> ProxyServer: 127.0.0.1
Service Providers -> ITSP Profile D -> SIP -> X_SpoofCallerID: Checked
Voice Services -> SP4 Service -> AuthUserName: anything
Voice Services -> SP4 Service -> X_RegisterEnable: unchecked
Voice Services -> SP4 Service -> X_ServProvProfile: D
Assuming GV is defined on SP1
Voice Services -> SP1 Service -> X_InboundCallRoute
ph,sp4(18667326184@tollfree.alcazarnetworks.com)
or to ring both phone ports
ph,ph2,sp4(18667326184@tollfree.alcazarnetworks.com)
Your phone will probably ring once when a call is blocked. There is a way to delay ringing, giving NoMoRobo time to answer before your phone starts ringing.
You can test NoMoRobo by defining a speed dial:
sp4(18667326184@tollfree.alcazarnetworks.com;ui=18005551212)
Change 18005551212 to a known good number and call NoMoRobo with the SD.
Now change it to a spam number and repeat.
Edit:
You can just turn on GV's spam filter or use both methods.