gainer

Etymology

From gain + -er. Compare German Gegner (“opponent, adversary”).

noun

1. One who gains a profit or advantage.
   … it is obvious that the Californian heiress is not the only gainer by an alliance which will enable her to make the easy and common transition from a Republican lady to a British peeress. 1892, Sir Arthur Conan Doyle, “The Noble Bachelor”, in The Adventures of Sherlock Holmes, HTML edition, The Gutenberg Project, published 2011
   Let every youth take a leaf out of my book and make it a point to account for everything that comes into and goes out of his pocket, and like me he is sure to be a gainer in the end. 1925-29, Mahadev Desai (translator), M.K. Gandhi, The Story of My Experiments with Truth, Part I, chapter xvi
2. One who puts on weight.
3. (sports, slang) A diving or gymnastics maneuver, from a high diving board or platform, involving a simultaneous inversion and rotation.
   I tried a full gainer with a twist, the way he did it, but succeeded only in smacking the water with my thighs. 1961, Stanisław Lem, chapter 4, in Return from the Stars