Maybe you knew that you could get a “secret” url from Google Calendar and use that one in iCal as described in this Syncing Google Calendar With iCal blog post. However, doing it that way it is a one-way sync from Google Calendar into iCal. To remedy this problem multiple external programs are available but they usually come with a price.
I had missed that it is now possible to sync both ways for free by following these quick instructions.
It works perfectly for me so far.