Sync Gmail Calendar and  iCal

Google Now provides an easy way to Sync your Gmail Calendar with your iCal Calendar using a Mac OS X application called Calaboration. If you’ve ever wanted to manage your Google Calendar from within the iCal interface, or vice versa, now you can without the need for an external expensive application such as Spanning Sync.

See Google’s detailed instructions on how to setup Calaboration:

Enable Google Calendar in Apple’s iCal