The URL needed here is the private XML address.
To get this URL, log on to your google calendar, go to settings, select the calendar tab and click on the calendar you wish to use for your hudson job. Then click the XML button at the bottom of the page in the 'private' section. It should pop up a window with the address.
The address generated will contain a 'magic cookie' number that you'll want to strip out.
The url will look like:
http://www.google.com/calendar/feeds/user%40gmail.com/private-9c22dffd59eaf9e5964e066f7ffff8f4/basic
and you will need to make it look like :
http://www.google.com/calendar/feeds/user%40gmail.com/private/full
To do so:
1. strip out the 'magic cookie' number after /private-
2. replace /basic with /full