Even when they try to support it, they changed the endpoint location (?! I guess even Google is bad at URLs), and their CalDAV implementation only supports the calls of a popular enough client they don't have control over.

"We have not yet provided a full implementation of all of the relevant specifications, however for many clients such as Apple iCal™ the CalDAV protocol should interoperate correctly."[0]

[0]: https://developers.google.com/google-apps/calendar/caldav/v2...