Google Calendar not appearing in gnome-calendar
Solution 1:
Simply logging out and in again solved the problem for me :)
Solution 2:
Google Calendar not working with the GNOME Calendar 3.20 app is a bug with the GNOME3 Staging PPA, as reported by WebUpd8.
If you want to use a more stable version of GNOME with probably fewer bugs, do not use the GNOME3 Staging PPA.