A gaoler is a jailer: the person whose job is to lock the doors and watch the people in a prison. This is the British spelling with gaol; the usual spelling now is jailer (jailor is another old form). It sounds slightly old-fashioned. Today people more often say prison officer or guard. Though actually, in older British stories the gaoler is often a figure at the door with a big key, not a modern officer with a radio — but the job is still to keep the door.

（英式拼法）獄卒、監獄管理員（現在較常說 prison officer）