Set it up as a new account in iCal preferences..."Google" should be an option in "Account Type". What this really does is set it up as CALDAV. Syncing should work right out of the box.
I remember seeing sync issues with some accounts in Mac OS X 10.5 "Leopard" that couldn't be resolved. Creating a new Gmail account for calendars fixed those.
First get the google calendar url from the web interface - it's in the calendar settings page. Then from iCal, click Calendar->subscribe and paste in the calendar URL.
Maybe you should tell us all the methods you have tried as well as your Mac model and OS version.