I realise that there are plenty of people who don't find smartwatches useful and who (with good reason) wouldn't want to use Google assistant. However, I have obtained a Mk.I Pixel watch to try out and I'm interested to see if I can make Google assistant work on it, despite such concerns.
Currently, trying to enable Assistant on the watch tells me to continue on my phone. On the phone, the Play Store opens in Firefox at the Google app page. I can then choose to open the Google app, and the process stops there.
Perhaps I've not granted sufficient permissions to Google's various apps. Or, it may be that this is something unsupported by GrapheneOS and will require a stock OS (I hope not). Or, something else. Does anyone know?