The Google Calendar team is proud to announce the public release of our support for the CalDAV protocol. You can now use Apple iCal with your Google Calendar, so you can work even when you're offline, sync almost instantly, respond to invitations from others and see the free/busy data of your friends and coworkers.
I've been looking in to this a bit deeper: the setup program they have is just that: a setup program. The actual work is done by getting the ical client to talk to the google servers using the calDAV protocol.
Basically one could add it easily in the preferences of ical if they'd tell you what to enter.
Basically: - the username is your gmail address - the password: well the one of your gmail account - the server url is https://www.google.com/calendar/dav/[gmail address URL encoded]/user (dots get through, you just need to replace the @ with "%40" (without the quotes)
The tool can recognize and setup other calendars ou might have (but that didn't work all that great for me).