The Collaborative International Dictionary of English v.0.48:

Gaoler \Gaol"er\ (j[=a]l"[~e]r), n.
   The keeper of a jail. Same as Jailer.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Jailer \Jail"er\, n. [OE. jailer, gailer, OF. geolier, F.
   ge[^o]lier. See Jail.]
   The keeper of a jail or prison. [Written also jailor,
   gaoler.]

WordNet (r) 3.0 (2006):

gaoler
    n 1: someone who guards prisoners [syn: prison guard,
         jailer, jailor, gaoler, screw, turnkey]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

GAOLER. The keeper of a gaol or prison, one who has the legal custody of the
place where prisoners are kept.
     2. It is his duty to keep the prisoners in safe custody, and for this,
purpose he may use all necessary force. 1 Hale, P. C. 601. But any
oppression of a prisoner under a pretended necessity will be punished; for
the prisoner, whether he be a debtor or a criminal, is entitled to the
protection of the laws from oppression.