They hope to make big 'gains' in Congress in the coming election. [=they hope that many more members of their party will be elected to Congress]
Her hard work 'gained' her their respect. [=she has gained their respect through hard work; they respect her now because of her hard work]
a stock that was one of the year's biggest 'gainers' [=one of the stocks that increased the most in value]
(..) - We'll all disappoint her before long. We need each other right now. After we're done needing each other... Oh, I don't want to fight you, Jorah the Andal. What do I have to gain? If I win, I'm the shit who killed an old man. If I lose, I'm the shit who was killed by an old man. (..)2016 TvShow: Game of Thrones Title: Book of the Stranger Season: 6 Episode 4