His best-known early works include the classic short stories "Old Faithful" (1934), which portrayed a sympathetic Martian scientist, and "Seeds of the Dusk" (1938), a haunting tale of alien spores arriving on a far-future Earth. Gallun later published novels including People Minus X (1957), The Planet Strappers (1961), and The Eden Cycle (1974), exploring themes of identity, immortality, and space exploration.
Although his prominence faded after World War II as science fiction evolved, Gallun's influence endured among writers and devoted readers. In 1997, he was honored posthumously with the first Cordwainer Smith Rediscovery Award, recognizing neglected science fiction authors of lasting merit and reaffirming his place in the genre's history.