In order to install Google Calendar with GNOME Software, launch GNOME Software,
search for Google Calendar and finally click the Install button.
a Linux terminal emulator,
type flatpak install nuvola eu.tiliado.NuvolaAppGoogleCalendar and press the Enter key to install this package.
Launch Google Calendar
You should be able to find Google Calendar in a list of installed applications. If not, you may have forgotten
to log out and log in and your desktop environment (Pantheon, GNOME, etc.) has not find out yet
that a new application has been installed.
Alternatively, you can launch Google Calendar from a terminal:
flatpak run eu.tiliado.NuvolaAppGoogleCalendar