Given a subscribed calendar with a url like
https://example.com/example.ics
To force Google Calendar to refresh and reload the contents right now, unsubscribe from the calendar and subscribe to a new calendar with a URL like
https://example.com/example.ics#1
Adding the anchor tag will force Google Calendar to think of it as a new calendar
Many people who come to this gist are not looking for a one time forced refresh but instead to refresh recurringly. If that's you, you should check out @derekantrican 's Google App Script
https://github.com/derekantrican/GAS-ICS-Sync
Which can refresh a calendar automatically every N minutes
Thanks to @rattrayalex for pointing people to this script
Check https://script.google.com/home/executions for errors in the log.
Apple is adding a bunch of additional data to the ics that is not always conforming to the ical standard.