If you haven't already, you will need to first check out: How do I sync my tasks to my calendar?
1. Open up your Google Calendar by going to http://www.google.com/calendar/
2. Click 'Add by URL' which is under 'Add' in the lower left corner of your screen:
3. In the following box, type in your unique 'Get It Done' calendar address. This link is located in your 'Calendar Sync' tab within your Get It Done settings. If you do not know where to find this, you should first read: How do I sync my tasks to my calendar?
And that's it! Your tasks should now show up in your Google Calendar. Google could be REALLY slow to update calendar feeds so you might have to wait for changes to be reflected on your calendar.