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On GNOME 3, there is a calendar default application. Why not allow to use gCalendar ?
It looks like there are a couple of pieces to this. The tool bar has an "Open Calendar" command, which may just be another gnome-open call. But it also has full calendar integration, showing appointments by day. That means integration with an ics download, or the Google Calendar Data API (http://code.google.com/apis/calendar/data/2.0/developers_guide_protocol.html).
Need to know more about GNOME's API for querying this.
GNOME 3 Google calendar integration code demo at https://github.com/jnns/gnome-shell-google-calendar/blob/master/gnome-shell-google-calendar.py