"Our gain is his loss."
Translation:Unser Gewinn ist sein Verlust.
Why isn't it "seiner Verlust"? Isn't 'Verlust' masculine and in the nominative case?
Yes, but possessive adjectives have similar endings to the indefinite article ein -- i.e. no ending in nominative singular for masculine or neuter nouns.
ein Vater, mein Vater, sein Vater, unser Vater; ein Kind, mein Kind, sein Kind, unser Kind
No -er or -es endings on any of those words.
In game theory and economic theory, a zero-sum game is a mathematical representation of a situation in which each participant's gain or loss of utility is exactly balanced by the losses or gains of the utility of the other participants. (Wiki)