Just been through this. It seemed like the install manual referred you to the Pilot guide which referred you to the install manual....
I tried the G3X team and ultimately contacted Guardian directly.
It is connected via a serial link on mine and CO Monitor is a selectable option on the RS-232 config screen.
On the normal G3X view, I seem to remember you had to go into the settings screen for the data bar and select "CO Level". This then appears on the data bar at the top of the G3X screen.
When you press the "Test" button on the CO monitor, you should hear a series of three beeps in your headset. This indicates it passed the self-test. If it fails, there will apparently be continuous beeping.
If you have a real-life increase in CO level, I believe the CO level flashes and a separate CAS warning appears on the G3X screen. However, my understanding is that there is no easy way to test that.
Hope that helps,
RV12 build in progress