Google have reported the problem as resolved. This reflects what we are seeing. We are continuing to monitor and ensure all synchronization tasks are processed.
Posted Jun 18, 2019 - 17:28 UTC
Update
We are seeing signs that most request to Google are now succeeding.
Posted Jun 18, 2019 - 17:15 UTC
Update
Google have updated their incident classifying it as a service outage, and also that they currently expect it to be fixed in about an hour.
We aren't seeing any change. We've taken steps to ensure service is maintained for other calendar providers and will ensure calendars are updated once Google have resolved the underlying issue.