Gardner was Brady's studio manager in DC until 1862, when they split, reputedly over a Brady's habit of attributing authorship of his assistant's work to himself, especially the battlefield photos. Gardner was the official staff photographer for Maclellan's army, and photographed at Antietam. After the war he travelled out west with the US Geologic Survey, surveying the route of the Kansas Pacific railroad. He closed his studio in 1871, and founded an insurance company. From the best of my understanding, he was most famous for A: his Lincoln portraits, including the last photos taken of Lincoln before his assassination, B: Antietam, and C: being the only photographer allowed at the execution of the Lincoln conspirators.