Google Calendar sync not working in Mountain Lion
I started encountering a similar problem with my Google for Business account last week. Mail.app was okay but Calendar refused to sync, returning the error message that "Your calendar URL is not a location that supports this request." every time Calendar tried to sync.

The solution was delete the account completely from System Preferences -> Mail, Contacts &amp; Calendars. Generate a new one-time password for the account (I use two-factor authentication on all my Google accounts). And the re-add the account in that System Preferences pane.

All is well now after doing this.