Microsoft updated some of it's Windows 8 built-in apps today, including Mail, Calendar, People and Messaging, bringing new features and performance enhancements but the big change with these updates is the removal of Google's Exchange ActiveSync support for Mail, Calendar, and Contact syncing.
Windows 8 users are displayed an error saying that "We can’t connect because Google no longer supports Exchange ActiveSync. Reconnect to get your emails and contacts using a different method. Cancel to save your email drafts and reconnect later."
You can reconnect your account to get back your emails and contacts but there is no way to get Google calendars back in the app, even if you previously had syncing calendars.
It was previously told that, if you've already connected your Google account to your PC running Windows 8 and Windows RT using the Mail, People or Calendar apps before January 30th, 2013, you don't have to do anything.