"Garp"; it sounds like the mating call of the Transylvanian sheep hound, but actually it is the marching cry of a new movement-at the moment, perhaps the most powerful four-letter word in the English language. The movement is a product of Madison Avenue hype, and it is hard to say what its goals are beyond motivating people to buy a particular book, but it has been doing that very efficiently.

Last night, novelist John Irving, inventor of the word but not of the movement, said "Garp" not once while speaking publicly for nearly two hours to an overflow crowd in the Coolidge Auditorium of the Library of Congress. He washed his hands of Garp in August 1977, when he handed the book to his publisher, and now he observes its progress with some detachment.

He did "Garp" once backstage, peering out through a peephole artfully concealed in the wall. "Look at all those kids out there wearing Garp T-shirts," he said. He was kidding; it was the usual kind of crowd you get at Library of Congress readings-serious, a bit academic, ranging widely in age, with more emphasis on youth and informality than the chamber music crowds you seein that auditorium. There wass not a single Garp T-shirt to be seen.

"You can't get them in Washington," expalined a fan at the reception and autograph party that followed the reading. "I think that's more than a New York thing."

The Garp T-shirt reads "I believe in Garp" on the front and "Do you believe in Garp?" on the back. Similar slogans are inscribed on bumpers, golf caps, sweatbands for head and wrist-all of which were launched by Pocket Books in April Fool's Day in a $200,000 promotion campaign for "The World According to Garp," and none of which could be seen at the Library of Congress.

What could be seen abundantly were some of the 1.2 million books which made up the first paperback printing of Irving's novel. This edittion is available in a choice of six colors and cover designs. The Washington crowd seemed to prefer the more subdued versions with relatively dark colors.

The four-letter name was invoked by poet William Meredith, who introduced Irving and remarked that, "I don't always have audiences like this for an introduction."

And he evoked the proper sense of awe with his summary of the Garp cult: "Garp lives and we are all believers. Tonight is like another Easter for our sect.

Following this garpful introduction,Irving ignored his earlier creation entirely and read instead from his next novel, "The Hotel New Hampshire."

He has written five chapters and parts of the sixth so far, as well, and an ending (which came first) and an epilogue. "The novel includes the rape of a child and the abuse of a homosexual, as well as countless racial slurs which seemed appropriate for reading in New York," he said.

"I can't say that I'm tired of writing that sort of thing, because I'm still writing it,. But I am tired of inflicting it on audiences, so I will read the first chapter,which is about a family of children growing up with a pet bear. I read it last summer at the Breadloaf writer's conference. It took three hours and 15 minutes, and some of the people who were at that reading are still asleep,"

He abbreviated the chapter, which is called "The Bear Called State of Maine" and deals, remarkably, with a bear named State of Maine. The reading involved some remarkable bear imitations, evoked frequent laughter and did not seem to put anyone to sleep.

World War 11 is also part of the chapter. He read one war episode lasting less than a minute and said it was the longest one in the book.

At the autograph party afterward, a remarkable proportion of the audience brought up hardcover copies of "The World According to Garp" to be autographed, demonstrating that they had not succumbed to ppaperback hype but to literary judgement (the hardover edition spent much of last year on best-seller lists and sold 115,000 copies).

Some of the dedicated hard core had copies of his earlier books, "The 158-Pound Marriage" and "The Water-Method Man," but nobody seemed to have his first one, "Setting Free the Bears," which has just been reissued in paperback.

"I let someone borrow my copy pf 'garp,'" said one fan who had Hemingway tucked under his arm. "I wish I had it here to be autographed." He added that he thought Irving "reads beautifully" and seems to have a thing about bears."

In a conversation earlier, Irving said he found he enjoyed having a popular audience after years of writing for connoisseurs, and that he didn't mind the frenetic promotion of his book "as long it doesn't involve me personally or take me away from my typewriter."

Besides bears, he has a thing about other animals. Yesterday afternoon, he said, he went to the zoo. "The white tiger was being fed and making a wonderful noise. The lesser panda was sleeping in a ttree." He said that he liked the lesser pandas better than their big cousins at the zoo-partly because they don't get so much hype. CAPTION: Picture 1, Author John Irving; Picture 2, 'Garp' author John Irving