Spanning Sync keeps your Google Calendar and your Mac’s iCalendar up to date with each other, eliminating the need for you to manage separate calendars. After you’ve set up Spanning Sync, changes made to Google Calendar will show up in iCal, and changes made in iCal will appear in Google Calendar.
Additional features of Spanning Sync [...]

GCALDaemon is a software package intended to keep your offline and online applications in sync with each other. That means if you use desktop applications like iCalendar, and also web-based applications like Google Calendar, GCALDaemon can keep them in sync and stop you from doing twice as much work to keep both systems up to [...]