I have done this, and its not working "right out of the box" I haven't made any changes just followed the bouncing ball.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.
It says the web address isn't validFirst 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.
I have tried the "follow the bouncing ball method" of adding my gmail account, it comes up as a CalDAV in the Account Prefs. Ive also tried to copy the Cal URL as suggested by ScoobyMcDo.Maybe you should tell us all the methods you have tried as well as your Mac model and OS version.