I wanted to import the Openshift.tv Google Calendar to my Nextcloud calendar app. The URL looks like this:

https://calendar.google.com/calendar/embed?src=redhatstreaming@gmail.com

A simple import in Nextclouds calendar app didn’t work, I figured I needed to get either an ICS or ICAL file, but couldn’t find any easy way. No button on the website, no knowledge base entries either.
By chance, I found a thread in Googles support forum which describes a way how to extract the ICS from a Google calendar:

1. Go to the settings for the Other Calendar, and note down the Public URL to this calendar, for example: https://calendar.google.com/calendar/embed?src=en-gb.christian%23holiday%40group.v.calendar.google.com&ctz=Europe%2FParis

2. Now take the part after ‘src=’ up to and including calendar.google.com so in this case our subscribed calendar ID is en-gb.christian%23holiday%40group.v.calendar.google.com

3. Now replace the ********** here with that text: https://calendar.google.com/calendar/ical/**********/public/basic.ics to give the final feed URL which you can download directly in your browser: https://calendar.google.com/calendar/ical/en-gb.christian%23holiday%40group.v.calendar.google.com/public/basic.ics

For the Openshift.tv calendar, this lead to this ICAL URL: https://calendar.google.com/calendar/ical/redhatstreaming@gmail.com/public/basic.ics