After the Terran Rebellion built their own version of the Defiant in 2372, Generals Julian Bashir and Zek turned the abandoned Klingon-Cardassian Alliance space station Empok Nor into a shipyard capable of producing up to twelve Defiant-class starships at once. The Capital Gain was the first of them to be completed and was placed under Zek's command.
Like the rest of the Defiant-class warships, the Capital Gain was equipped with a Romulan cloaking device. After Empok Nor and the other Defiant-class ships were destroyed by the IKS Ya'Vang, the Capital Gain was captured and its cloaking device fell into the hands of the Alliance. (DS9 - Mirror Universe novel: Saturn's Children)