I think it is easy to understand why a CCW license holder might carry even when a GFZ sign is posted: the entire purpose of carrying a firearm is to provide for self defense and defense of one's family. The need for a gun does not end merely because some anti-gun property owner decided to post a sign. I think that most gun owners want to obey the law, except when that law puts them into excessive danger.
Just as some good citizens knowingly broke the law by carrying even before concealed carry became legal, some will carry where it is technically prohibited. Leaving a firearm in an easy-to-break-into vehicle is not a great solution. Leaving a firearm at home because for one small part of a day you will not be able to avoid entering a business that is posted is not an acceptable compromise.
I would guess that most legal gun concealed carriers honor the property owner's decision where possible by not patronizing that business. But where it creates a dangerous or overly inconvenient situation you can just make sure that concealed means really concealed. I'll bet that if a GFZ business owner is being threatened with death by a criminal, and if a licensed gun carrier intervenes successfully, the business owner is not going to get too upset that his GFZ sign was ignored.