OFFICER WAS ATTEMPTING TO CLOSE AND LOCK THE PUBLIC COAL LOAD-OUT FACILITY, LOT A LEFT GATE. LEG WAS STUCK IN THE GROUND AND OFFICER PULLED ON THE GATE, HAND SLIP, & CAUSED OFFICER TO FALL BAC KWARDS, FELL & HIT HIS HEAD ON GRAVEL ROAD ROCKS& SUSTAIN SMALL CUT ON BACK RIGHT SIDE OF HEAD. OFFICER GOT DIZZY FOR A SHORT WHILE & GOT A HEADACHE.