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!

You could check out this page by Google, explaining about CalDAV used by your iDevice.

In case you have two-step-verification, you need an application-specific password as explained on this Google page. I think this - the two-step-verification - is the issue you're having.

I had the same problem, some Gmail calendars were synced and some not.

For me the solution was to check if they are marked for sync within Gmail or not.

To that end visit Google Calendar Sync Select and check if your problematic calendar is marked for sync or not.

By default, all of your own calendars and birthdays are enabled to sync. All others are disabled by default.