Looking for some help in completing the bypass process. I have the Z3 installed and using the GMOS module for Onstar in an '08 Enclave. When it was installed, the GMOS harness was connected to the Parking brake sensor wire on the Z3 harness. Do I disconnect those two wires, tape off the one coming out of the GMOS unit and ground the Z3 wire as part of the two wire bypass?

That is correct.

depending on the vehicle, you may need to use a relay on the GMOS module for your acc