Please look in ~/.cache/msyncd/sync/logs/ for files called “Google.Calendars-XX.log.xml”, and post them here.
Also for collecting detailed logs, follow CalDAV and CardDAV Community Contributions | Sailfish OS Documentation
There’s also lots of info (albeit dated) at this thread: Google Calendar does not sync - #41 by jovirkku
Finally, this may be a duplicate of this:
Google calendar doesn't sync automatically which is known and tracked by Jolla for 4.4+