The outdoor areas of forest preserves (other than Cook County) are not prohibited areas. Period. So the answer to your question is yes. Only private entities can create GFZ's by posting places that are not on the list of prohibited areas. "All firearms, including concealed firearms, are also prohibited on all IDNR State Refuge areas, IDNR Dedicated Nature Preserves,..." http://dnr.state.il.us/law3/Documents/ConcealedCarryQnA.pdf