I have been syncing Mac OS contacts (Address Book) with Google for a long time. For some reason, though, that option is now grayed out in Contacts preferences.
Why? How do I get it back?
Most likely because you have Contact syncing via iCloud turned on. You can turn this off using the check-box in Preferences > iCloud.
I don't have iCloud syncing for Contacts and can sync via Google, but when I turn iCloud syncing on it warns me that Google syncing will be turned off...