Your post is somewhat ambiguous, so I'm not sure that I'm answering the right question.
If a call comes into the OBi (on landline, GV, SIP, or OBiTalk) and is forwarded to an external PSTN number via GV, then it's not possible to display the original caller's number to the forwardee; this is a restriction of GV. You could forward via a different provider that permits "spoofing" the caller ID, though I'm not aware of any free ones. Anveo will work as you desire; it costs $0.01/min. to US or $0.005/min. to most of Canada. Simply set X_SpoofCallerID in the OBi and set Outbound CallerID to "CallerID from SIP Device" in Anveo. There are lower cost options, but a more complex setup is required.
Alternatively, it may be possible to set up the provider handling the original call to do the forwarding, rather than using the OBi. Such a setup will generally preserve the original caller ID, though it obviously depends on the provider whether the desired condition will be available, and what charge, if any, applies to the forwarding leg.
If you describe your application in more detail, there may be a better option.