The Collaborative International Dictionary of English v.0.48:

Gainer \Gain"er\, n.
   One who gains. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

gainer
    n 1: a person who gains weight [syn: gainer, weight gainer]
    2: a person who gains (gains an advantage or gains profits);
       "she was clearly the gainer in that exchange"
    3: a dive in which the diver throws the feet forward to complete
       a full backward somersault and enters the water feet first
       and facing away from the diving board [syn: gainer, full
       gainer]

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "gainer":
   belly buster, belly flop, belly whopper, cannonball, crash dive,
   dive, drop, fall, header, jackknife, nose dive, parachute jump,
   pitch, plunge, pounce, power dive, running dive, sky dive,
   stationary dive, stoop, swan dive, swoop