Below is how to see (read-only) your Nextcloud calendars in Google Calendar. I do not believe Google supports writing to Nextcloud calendars from Google Calendar. This example is for my Calendar “Example Calendar”.
From the calendar app page:
Under your list of calendars on the left
- Click the share button
- Select “Share Link”
- The click the second share button
Your browser will open to a new Nextcloud Calendar page that is for public viewing.
https://your-nextcloud-server.com/index.php/apps/calendar/p/XXXrandomlettersXXX/Example-Calendar
On the top right, click on the Subscribe button.
Your browser will either ask you how to handle this type of link, or will redirect you directly to Google Calendar, where you can add your calendar in this format.
webcal://your-nextcloud-server.com/remote.php/dav/public-calendars/XXXrandomlettersXXX?export
Man and Keyboard 
hello, thanks for this howto, is there any way to force google to check periodically for changes in the imported calendar? I can succesfully share my nextcloud calendar to my friends – they see everything in my NC calendar when they import it to their google calendars, but when I add new events they can’t see them (unless they remove and add the calendar again…)
For me, Google refreshes at a quick enough rate that I have not noticed a delay.
It looks like it works now, but two months ago, changes which were made in NC calendar (after sharing the calendar to Google) were not displayed in Google calendar. Strange. 🙂