Gets the unique iCalUID of the event. Note that the iCalUID and the event id
used by the Calendar v3 API and Calendar advanced service are not identical and cannot be used interchangebly.
Which isn't exactly true. GetId() returns the API id, but just with a @google... tagged on the end if there is no iCalUID available, so you can extract an ID which can then be used with the Calendar API/Advanced service.
I've only seen the iCalUID returned when the Google calendar events have been imported from another iCal compliant calendar.