As you can probably understand, the Simonics GVGW is a gateway, meaning, it is connecting and passing traffic between two different systems. On one side is the old Google Chat/XMPP protocol. You authorize GVGW to access the Chat service on your Google account, via OAUTH 2.0 authentication, when you follow the instructions to set up the gateway. Let's call that the GV side. Entirely separate from that, you are issued a SIP userID and password. You then enter that information on your OBi, and the OBi is simply using a standard SIP service provider. The OBi doesn't know nor care that it's a middleman for GV. Let's call this the gateway side.
The unauthorized error simply means that you've incorrectly entered the gateway user ID and password onto the OBi form, or else you are doing this on an OBi that is somehow still trying to use whatever old configuration parameters it has.
Go back to the GVGW web page. Click the link to change the password. Do NOT refresh or reload that browser page, as it will change the password again! Enter your 1xxxxxxxxxx GV phone number and GVGW SIP password on the OBiTALK form.