The Collaborative International Dictionary of English v.0.48:

Galloper \Gal"lop*er\, n.
   1. One who, or that which, gallops.
      [1913 Webster]

   2. (Mil.) A carriage on which very small guns were formerly
      mounted, the gun resting on the shafts, without a limber.
      --Farrow.
      [1913 Webster]

   Galloper gun, a light gun, supported on a galloper, --
      formerly attached to British infantry regiments.
      [1913 Webster]