For more informations on google calendars, please visit the Google Help Center
1. From a Google Calendar go to the calendar you want to share and click on Settings and sharing.
2. Click on Calendar Settings and tick the "Make available to public" checkbox.
3. Then choose the Integrate calendar tab and copy the "Public address in iCal format" link.
4. Paste this exact link on the Artist defaults - ICal links - External Calendars section.