Can't enable Gmail calendar syncing
After a ton of trial and error, I finally got this working. I don't know what the heck the issue was, but it was a total mess. At one point, I was somehow getting reminders for events in Notification Center even though they weren't appearing on my calendar.

Anyway, I had to:


Turn off ALL calendar syncing, including for other accounts (iCloud, Facebook, etc.)
Completely delete the problematic account from System Preferences > Internet Accounts
Completely delete the contents of ~/Library/Calendars**
Log out and back in


Then I set up the problematic account again from scratch (in System Preferences > Internet Accounts) and enabled calendar syncing. This time, it worked like normal, and then I was able to reenable calendar syncing for all my other accounts.

I didn't have to change any settings or anything on Google's end (google.com/calendar).

** Note: I was only able to do this because I don't store any calendars locally. If you have local calendars, deleting the contents of this folder will delete all those events. Just a warning!