If you have upgraded to Joomla 1.6, the current release is NOT native to 1.6 and you will get errors. Wait for the update to 2.3.X
At the moment GCalendar is in beta stage, which means it can have some errors. But it is 100% joomla 1.6 native. Please use the G4J forum to solve your joomla 1.6 problem.....