Remove Google Sync and restart BB.
Remove all email accounts.
Re-add email accounts.
On you BB go to:
OPTIONS
ADVANCED OPTIONS
DEFAULT SERVICES
Make sure your gmail calendar is default.
Now reinstall Google Sync.
Some helpful tips here — http://www.google.com/support/forum/p/Google+Mobile/thread?tid=341e210ba2f3e9a5&hl=en