He had congestive heart failure and died Tuesday, the family said.
Gray developed the 1979 "Syndrome" story after reading books and interviewing scientists about the dangers of nuclear power, the Los Angeles Times said, adding the film's timeliness was unexpected.
Twelve days after it opened, the Three Mile Island power plant in Pennsylvania had a partial nuclear meltdown.
"The Chine Syndrome" -- starring Jack Lemmon, Jane Fonda and Michael Douglas, and directed by James Bridges -- also examined trends in broadcast journalism and corporate responsibility that complicated efforts to report the nuclear disaster accurately.
Newsweek called it "a rare phenomenon -- a piece of popular entertainment that immediately foreshadows a major news event and then helps explain it."
The screenplay won the 1980 Writers Guild of America Award and received four Academy Award nominations, including best original screenplay, best actor (Lemmon) and best actress (Fonda).
For television, Gray helped develop the 1986-87 series "Starman," and was a writer and producer of the 1988-89 season of "Star Trek: The Next Generation."
Gray's books include "The Warning," about the Three Mile Island accident, and "Drug Crazy: How We Got Into This Mess and How We Can Get Out," an indictment of the U.S. war on drugs.