The Collaborative International Dictionary of English v.0.48:

Grumbler \Grum"bler\, n.
   One who grumbles.
   [1913 Webster]

WordNet (r) 3.0 (2006):

grumbler
    n 1: a person given to excessive complaints and crying and
         whining [syn: whiner, complainer, moaner,
         sniveller, crybaby, bellyacher, grumbler,
         squawker]