for updated instructions, please see this post
how to sync your gmail calendar and ical
this one is pretty easy.
just open ical, goto preferences and accounts.
hit the + button
fill in your google username,
eg bob.smith@gmail.com
and password and it will get the rest by itsself.
you then need to put in the url of your calendar in the server settings tab.
the url looks like
https://www.google.com/calendar/dav/username@gmail.com/user
have fun.

Pingback: how to stop your google calendars appearing as delegates in ical - Jonathans blog
Pingback: new year! (geting re-orgainsed, syncing todo lists between mail.app, ical and iphone) - Jonathans blog