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]