It's mandatory as some apps only offer the use of google calendar. If we want to fully benefit from the encrypt proton tech., we have to obtain this continuous sync.

Bi-directional continuous synchronisation please!

+1

I use Proton Calendar for my personal account and I have a Google Calendar for work related stuff.

The Google Calendar cannot be public, so I cannot share an ICS to sync the events with Proton.

In my case I would be happy to simply be able to view the Google events in my Proton Calendar, I do not need to be able to also edit them from there.

Integration with google calendar to allow me to see my wife's calendar entries inside proton calendar. Also, a way to mark a calendar entry in proton calendar to be synced with my google calendar so my wife can see it.

Have time planning tools for service techs in multiple Google Calendars - a setup that I cannot change without major impact on daily business.
Need to have live integrations to migrate admins to Proton

downloading an ics file is helpful for moving my calendar events from google to proton. It is not helpful for keeping a shared calendar with my wife who uses gmail.

+1 on this. I'm a member of a few organizations who mostly use the Google Suite, and so I'd love to the ability to get bi-directional sync, as I can't navigate manually two calendars (others need to see my availability).
Before we have such feature, the Proton Calendar is useless for me, unfortunately.