After watch app is installed. Open it. Give it permissions. Watch will say "Companion app is not installed. Check your phone". The phone will light up and the Google Play Store will open to the Gassist companion app install page. Click install. Click open. Give it permissions and follow the authentication instructions.
It works well for just asking general questions, but unfortunately for me the most important for a watch assistant is to be able to set reminder, alarms, send messages, etc, which does not work with GAssist, which doesn't communicate with the system.