Default Gmail calendar items are not displayed in any calendar app

My main Gmail account is fully synced with my device.

In all calendar apps, no appointments from it (and any other gmail account) are displayed (checked with stock calendar, CAL, widgets etc.)

My Gmail calendar is not even displayed in the list of available calendars in any app.

Of course, all my appointments are there on the web version of Google calendar, and all are fully synced.

Any advise to remedy that annoying issue would be very welcome.