Well for what it's worth, I was having the same problem. I am not using the iCloud sync but was just trying to sync my Mountain Lion version of iCal with my Google calendar. I already have my iPhone syncing nicely with Google just fine. So it was quite frustrating that this wasn't working.
Suspecting that it might simply be a configuration issue, I deleted all the iCal calendars and completely started over by doing the following.
1- deleted all iCal calendars
2- quit and restarted iCal
3- navigated to iCal preferences -> Accounts tab -> click the "+"
4- chose Account Type = "Google"
5- entered my complete Gmail email address, e.g. "firstname.lastname@example.org"
6- entered my password
7- clicked Create and crossed my fingers
Seems to be working now without the annoying error messages constantly popping up. For the record, I hadn't included the "@gmail.com" before, and the time before that tried Automatic as Account Type. Seems a silly thing, but perhaps this is where the error was occuring. Good luck!
Also, I have a few sub calendars assigned to my Google calendar and was able to sync them as well by using the "Delegation" tab on the Accounts tab of iCal preferences.
I didnt have a problem with automatic BUT
I found if you tried to delete a calendar which you nolong want anywhere ical after a "reload" using these very helpful guidelines, that ical will "accept" the command then start pumping out these messages again...
All admin in terms of deleting calendars require you to go to a broswer and mange them there....
The fundamental problem here is once your have commited some command like delete a calandar in ical which google WONT accept form this client them messages keep coming because it keeps trying.
Stupid Robot you cant stop OTHER than wiping the calendar and starting again as per these instructions...
Thanks all helpful discussion and ical is working properly again