Coppel's first science fiction story was "Age of Unreason" (1947) in Amazing Stories. Other short stories include "The Dreamer" (1952) about a man called Denby, who wants to be the first to orbit the moon, published in The Magazine of Fantasy and Science Fiction and reprinted in the anthology Best Short Shorts, (1958) edited by Eric Berger. His 1960 post-holocaust novel "Dark December" describes the aftermath of nuclear war.

As Robert Cham Gilman, he wrote the "Rhada" sequence of science fiction novels aimed at the young adult market. These space operas set within a galactic empire comprise: The Rebel of Rhada (1968), The Navigator of Rhada (1969), The Starkahn of Rhada (1970) and a prequel called The Warlock of Rhada (1985). Another story in the same genre can be found under his own name in Brian Aldiss's collection Galactic Empires.

The Burning Mountain: A Novel of the Invasion of Japan, (1983), is an alternate history depicting what could have happened if the United States and its allies had been forced to invade Japan in 1946, had the Trinity test of the Fat Man nuclear design tested on July 16, 1945 failed. This is based on the Operation Coronet and Operation Olympic United States battle plans for the invasion of Japan, which were rendered moot by Japan's surrender after the atomic bombings of Hiroshima and Nagasaki.

In 1974 he had a bestseller with the suspense thriller Thirty-Four East about the Arab-Israeli conflict. Another political thriller was The Apocalypse Brigade, 1981, about the United States at war with global terrorism.