I'm seeing shared Google calendars on my Jolla, and I haven't done any special configuration. I just created a Google account, and shared calendars were instantly visible in Calendar > Manage calendars.

No idea why it's working for me but not for you.

Update: There's discussion on this in together.jolla.com...
https://together.jolla.com/question/...olla-calendar/
https://together.jolla.com/question/...-the-calendar/

With https://www.google.com/calendar/syncselect, you should be able to select which calendars are synced via CalDAV. But according to the discussions linked, that isn't necessarily the solution to this problem.