I read online - right click on the calendar to export it.
But in Gnome Calendar 46.1 right clicking eithre the calendars in the list on the left or the dropdown for the list of calendars - i couldn’t find the exporting option.
That’s in Evolution’s GUI, not in GNOME Calendar’s.
In GNOME Calendar 49, you will be able to export individual events, as it was recently implemented. The ability to export entire calendars is not yet merged.