The OBi200 has a feature allowing an SIP device to register to it in a similar way as registering to an SIP service provider. The link I provided shows how to define SP4 with this feature. The change to SP1 will route inbound calls to the IP phone. You need to use OBi Expert to make these configuration changes.
To enter OBi Expert from the Dashboard click on the gear ICON to the right of your OBi200 then select OBi Expert Config, confirm and select Enter OBi Expert.
To navigate click on the titles with the + then click on a sub-title.
An example: click on Service Providers then click ITSP Profile A General.
To make a change BOTH boxes to the right of the value must be unchecked.
When all changes are complete on this page, click Submit at the bottom of the page.
OBiTalk will download the configuration changes to the OBi and reboot it.
Wait until this process is complete before continuing to make another change on a different page.
If you go this route and get one phone working, I'll show you how to define the 2nd. GV only allows 2 simultaneous calls, so only 2 devices can be used at the same time. You have the option to add a 2nd GV account.
The only other method I know to use GV on an IP phone is Simon Telephonics Google Voice Gateway.
https://simonics.com/services/I have not used this version of the Gateway, so you need to do your own research. If you decide to use the GW, you should probably convert your OBi200 to the GW making the setup just as complex or more.