A FAIRCHILD SA-227 WAS POSITIONED BY GROUND PERSONNEL IN A PARKING SPOT ADJACENT TO ANOTHER OPERATOR'S ARRIVAL GATE. DUE TO OTHER AIRCRAFT IN THE PARKING AREA, THE TAIL OF THE FAIRCHILD ENCROACHED INTO THE SAFETY AREA OF THE NEXT GATE. AS AN ARRIVING LOCKHEED L-1011 WAS TAXIING INTO ITS GATE AREA, GROUND PERSONNEL NOTICED THAT THE AIRPLANE'S LEFT WING WAS GOING TO CONTACT THE VERTICAL STABILIZER OF THE FAIRCHILD. GROUND PERSONNEL DID NOT SIGNAL A STOP IN TIME TO AVOID THE COLLISION.