FORREST GUMP. By Winston Groom. Doubleday. 228 pp. $14.95.

GENTLE GIANT Forrest Gump of Mobile is 6-foot-6, weighs 250 pounds, and has an IQ of 61, but the local high school football coach, knowing a refrigerator when he sees one, pulls strings to get him out of the "nut school" and onto the home team. Forrest doesn't understand the game -- his mother has to tie his neckties for him -- but he makes a lot of touchdowns anyway because "I run a lot faster when I'm bein' chased. What idiot wouldn't?"

On to the University of Alabama to play for the Crimson Tide under Bear Bryant, who enrolls him in courses taught by complaisant faculty members known for passing football players. One of these is a graduate physics course called "Intermediate Light" in which Forrest gets an A and is diagnosed as an "idiot savant" by the dean of the medical school. Unfortunately, he flunks Remedial Freshman English, a course so easy that the Bear did not even think to get a complaisant professor for him. When he also flunks Phys. Ed. he is expelled from the university in disgrace and drafted into the army.

The saddened Forrest, who was looking forward to taking "Advanced Light," finds himself in the grip of a drill sergeant who says things like: "I want half of you to go over there and half of you to go over here, and the other half of you to stay put." Instead of following through on its slogan of "Be All You Can" and letting Forrest design new rockets from his own original equations, the army names him company cook. Commanded to make a stew, he panics and throws all the food in the kitchen into the biggest pot he can find -- the gas boiler -- and turns it up full blast. The resulting explosion blows the roof off the mess hall, the dishwasher through a wall, and everyone's clothes off. The terrified Forrest, wearing nothing but his tall chef's hat which has miraculously stayed put, takes off and runs naked through the Officer's Club.

Next comes Vietnam and a view of America's noble experiment through the eyes of a certifiable idiot. Here author Winston Groom, who with Duncan Spencer has written eloquently about that conflict in the nonfiction Conversations With the Enemy, turns serious and gets in some wrenchingly simple observations, as when Forrest comes upon his dead comrade and thinks, "He was still holdin' to the gun for dear life, which he does not have any more of now."

After a one-man assault on the Vietcong in which he is wounded in the buttocks, Forrest is awarded the Congressional Medal of Honor and goes to the White House to receive his medal from the President. After the ceremony LBJ shows him his gall bladder scar and asks, "Where were you wounded, son?" Anxious to be agreeable and literal- minded as always ("If he axe me, he must want to know"), Forrest drops his pants and presents his wound to his Commander-in- Chief.

Having learned ping-pong in the army hospital, Forrest is now sent to China to play for the U.S. team. Told to present himself at the Peking Duck restaurant for a banquet in his honor, he loses the slip of paper containing the restaurant's name and address in Chinese and has to make himself understood to the cab driver in body language. Flapping his arms in imitation of a duck, he is taken to the airport instead.

NASA gets wind of his computer-like brain and decides that an idiot savant is just what they need for their next space flight, so Forrest suits up and joins his galactical companions, a female Air Force major and an orangutan named Sue. The headlines -- "Woman, Ape and Idiot in Next U.S. Space Effort"; "Girl, Goon, and Gorilla to Lift Off Today" -- hurt Forrest's sensitive feelings. Only the New York Times is tactful: "New Space Probe has Varied Crew." he story, up till now a superbly controlled satire, takes a nose dive into broad farce when the space capsule crash-lands in a never-emerged nation of cannibals whose chief was once an exchange student at Yale. The tough lady major goes native a la Tondelayo while Forrest and his simian pal take up Peace Corps duties until they are rescued and returned to Washington for a meeting with President Nixon, who tries to sell Forrest a used wristwatch.

Next comes professional wrestling, with Forrest billed as The Dunce and sent into the ring wearing a dunce cap and a diaper. On to Hollywood and the title role in The Creature from the Black Lagoon oppposite a famous actress. Here matters become not only incredible but unwise, unless the author has secured a release, for despite the usual disclaimer -- "Any resemblance to living persons or actual events is unintended and entirely coincidental" -- the famous actress can easily be construed as Raquel Welch, into whose mouth Groom puts some very foul language, especially in the scene when she runs naked across Wilshire Boulevard after losing her clothes in a typical Gumpian mishap.

FORREST RETURNS to Mobile, builds a multi-million-dollar shrimp business, and is asked to run for the U.S. Senate. "But I'm an idiot," he protests. "You'll fit in perfectly," says the party chairman.

Which all goes to prove that you don't have to be an idiot savant to figure out this roman ,a clef. You do have to exercise patience. Groom writes in the first person from Forrest's viewpoint, so we get idioticisms piled on top of Southernisms ("bidness" for "business") that make for extremely hard reading at times. He also lets people come out of character. Dan, the legless Vietnam vet Forrest meets in the hospital, goes inexplicably and disappointingly from being a bitter philosopher to a wacky Gumpian sidekick, rather like a Laugh In version of Ratso Ritso or the Peter Sellers version of Toulouse-Lautrec, while the orangutan acquires so much humanity that he rides on Greyhound buses without attracting attention. Perhaps Groom intended this as a serious statement or symbol, but it comes through as King Kong footage.