After installing the app, Google starts to save the events from your calendars to you computer. To customize the list of calendars that are available offline, click the wrench icon, select "Offline" and then "Offline settings".
When you are offline, most Google Calendar features are disabled, just like in the Gears implementation. You can't create new events, edit the existing events, search your calendars or print your calendars. You can only view some of the events from your calendars and RSVP to existing event invitations. Google Calendar doesn't even save all the events from your calendars. When trying to find older events, Google Calendar showed this message: "Your offline calendar only contains events from Jul 21, 2011 to Jan 5, 2012".
Google says that you can only use offline Google Calendar for one account, even if you use multiple sign-in. Google Apps users can try this feature, but only if the admins have switched to the new infrastructure. "To disable your offline access, click the gear icon and select Disable offline calendar from the drop-down menu. This will disable offline access and remove any data stored on your computer."
While it's nice that the offline Google Calendar is not a separate app, the offline mode is read-only and doesn't even sync all your events. Google should let you search, create and edit events and add support for other browsers.
{ Thanks, Scotty. }