You should check out Google Assistant SDK - Home Assistant (home-assistant.io) if you are still looking for the answer to this.
I setup some buttons for a Tuya Fan that I can control from Google Assistant on my phone but with the new Tuya integration I cannot get to work without paying their subscription.
once you have followed the few simple steps. you can use NodeRed or even just create a template switch like this:
- platform: template
switches:
dining_room_fan:
friendly_name: "Dining Room Fan"
icon_template:
mdi:ceiling-fan
turn_on:
service: google_assistant_sdk.send_text_command
data:
command: turn on Dining Room fan
turn_off:
service: google_assistant_sdk.send_text_command
data:
command: turn off Dining Room fan