Now that Forrest Gump rivals the O.J. Simpson trial as a national obsession (T-shirts, hats, with cookbooks and possibly Ping-Pong paddles to come), Pocket Books has asked author Winston Groom to write a sequel to his 1986 novel, which has been selling briskly since the film's release. ''It won't be Forrest Gump in the Temple of Doom,'' says Groom. ''I'm not trying to do some kind of exploitive thing. It's going to be Forrest's story where it left off in the book, in the late '70s. That gives me 15 years to make fun of people.'' But don't count on seeing the next chapter in Gump's life on the screen. Though the film is expected to be a front-runner at next year's Oscars, and insiders predict it will pass $250 million at the box office, a sequel seems unlikely. The movie's writer, Eric Roth, doesn't think either Tom Hanks or director Robert Zemeckis will return to the Gump pump. The film, says Roth, ''is what it is. You could do [a sequel], but it would be a bit whorish.''