That doesn’t work for Google Cast devices. My typical Roon environment consists of different GCC’s only – GROUP ZONES is greyed out/non functional. You have to use the Google Home app to group Google Cast audio devices as documented in: Chromecast
Roon supports grouped playback on audio-only Chromecast devices via the Google Home app. Using the Google Home app, you can set up groups of devices, which then appear as additional audio devices in Roon alongside the hardware devices. So for example, you could group a few Chromecasts into a “Whole House” or “Living Room + Patio” zone, enable them as separate Zones in Roon, then play to them freely. Note that video Chromecast devices cannot be grouped together in this manner.
Due to hardware limitations, Chromecast devices can not be grouped with other, non-Chromecast devices outside of the Google Home app.