Source : Webster's Revised Unabridged Dictionary (1913)

Gainer \Gain"er\, n.
   One who gains. --Shak.

Source : WordNet®

gainer
     n 1: a person who gains weight [syn: {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: {full
        gainer}]