It turns out that the newly updated Mail, Calendar, People and Messaging app for Windows 8 has removed support for Google Calendars entirely, pulling the ability to sync both new and existing calendars due to Google pulling the plug on Exchange ActiveSync support. Upon attempting to use the updated app you'll receive a message similar to the one below, instructing you to reconnect your account.
We can't connect to [address]@gmail.com because Google no longer supports ActiveSync. Reconnect to get your emails and contacts using a different method. Cancel to save your email drafts and reconnect later.
You'll notice that while you can reconnect your account to get back your emails and contacts, there is no way to get Google calendars back in the app at this stage, even if you previously had syncing calendars. Microsoft previously stated that EAS-syncing Google accounts added to Windows 8 before January 30th would have contacts, emails and calendars "continue to sync properly", but this no longer appears to be the case.
It remains to be seen how Microsoft will support Google Calendar in the future through the stock Calendar app in Windows 8 and Windows RT, but for now there appears to be no support to speak of.