Definitions of gainer:

noun: 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
noun: a person who gains (gains an advantage or gains profits)
Example: "She was clearly the gainer in that exchange"
noun: a person who gains weight
name: A surname (rare: 1 in 50000 families; popularity rank in the U.S.: #6702)