I don't know if you found the solution yet. It is working on my iWatch.
Go to settings, accounts an add your gmail account with at least the calendar option. Then go to the standard calendar and go to the settings of the calendar. Choose here only the Google Calendars.
Then go to the iWatch app on your iphone and choose calendar. There you don't choose match Iphone but use instead on both settings use the adjusted options and on the calendars choose only the Google Calendars.
So there is no iWatch app for Google Calendar but with this workaround you still can show them. It works fine with me!!