I'm leaving this here in case it helps anyone else having spent several hours trying to work out how to fix the issue. New Moto g5 plus. Not only would Calendar not sync, but I couldn't add events to the phone. It would say it was added, it appeared in my gmail account but it simply would not come up on the phone. The solution. Settings > Apps > Choose Calendar. Force Stop and then disable. Then reload Calendar so that you have version 5.6.6-137495241-release running. Then disable Calendar from auto updating (go to play store, my apps and games, choose Calendar and make sure auto update is off for that specific app). If you don't do the last step, the calendar will eventually update and - woosh, your appointments no longer will be appearing on the phone.
Solved Calendar problem Moto g5 plus/Nougat