I had the same problem. You can follow the steps below from this fine article from The Verge to fix it. Thanks Verge! Unfortunately you can’t use the built in iOS google setup to sync to your calendar because Google has increased security. You could ask Google to lower the security in your account on a per device basis, but I wouldn’t recommend that.
Part 4: Syncing Calendars with CalDAV
The steps required to integrate Google Calendar with iOS are very similar — go to Settings and navigate down to Mail, Contacts, Calendars once again. This time, tap Add Account… and select Other, and then Add CalDAV Account. Just like above, you’ll enter “google.com” for the Server field, your user name and password, and add an optional description. After clicking Next, navigate back to the Calendar app and the entries should appear shortly. To sync multiple calendars, go to this page and select the calendars you want to sync, then click Save. If you run into any problems, double check that SSL is enabled in both your CardDAV and CalDAV accounts.
Several suggestions. rventura said “I solved the problem by deleting the “~/Library/Calendars/Calendar Cache” file.”
Then do Glynns sugggestion “It got iCal working again, but the Google calendar still wouldn’t work properly. Then I deleted the .caldav subfolders that are in the same folder, after which iCal at first froze up and had to be forced to quit. Now, having re-started iCal, the Google calendar seems to be working just fine. Weird. “