I just upgraded from 15.10 to 16.04 and also upgraded GNOME to 3.20 and my Google calendar isn't showing events & isn't listed in settings either.
The Google account is set up correctly in Online Accounts and everything else integrates properly (documents, photos, contacts, files etc).
This screenshot shows Calendar telling me there isn't a Google account (click to set up) but having clicked you can see that the account is already there...