Not yet compatible with Leopard's iCal 3.0.
OS-independent Java program that offers two-way synchronization between Google Calendar and various iCalendar-compatible calendar applications. GCALDaemon is primarily designed as a calendar synchronizer but it can also be used as a Gmail notifier, Address Book importer, Gmail terminal and RSS feed converter.
For details on Leopard compatibility, see the SourceForge forum thread.