When I add google calendar as external calendar into Zimbra web client, using ICS, Zimbra prompts the following error:

Appointments could not be retrieved. make sure the following URL is a valid ICAL feed: https://www.google.com/calendar/ical/xxxx/basic.ics

When I add google calendar as external calendar into Zimbra web client, using XML, Zimbra proceed without prompting any error. However, the calendar is completely empty.

Anyone manage to do this successfully without making your google calendar public?