I wonder, can create another gmail account with another GV number and forward to that number?
You can create a second GV account and set up your OBi to use it instead of your main GV account. That will allow you to make free calls with the OBi, albeit with the "wrong" caller ID. Then, add another number to your main GV account that rings the OBi, e.g. an IPKall DID pointing to a free Callcentric account, to which your OBi's SP2 would register.
With this setup, calls to either GV number would ring the OBi, though only calls to the main one would ring the computer.