Gatekeepers are mentioned quite a bit in the Old Testament.  There were both gatekeepers of the temple and gatekeepers of the cities.  The city or temple would be surrounded by a wall.  And the gate was the only way in or out.  The gatekeeper would be tasked with controlling who went in and out.  They were responsible for keeping the bad out and letting the good in.  Gatekeepers were vitally important to OT life.  If a gatekeeper was not vigilant, he could allow the destruction of the entire city.