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]
(..) The insurance company won't pay unless the murderer is caught. And if it turns out to be me... ...they won't pay at all. Do you understand? But you stood to gain from Gluant's death, didn't you? Gain? What would I gain? Gluant's share of the restaurants. Oh, the restaurants were a disaster. (..)2006 The Pink Panther