FRESNO, Calif., Jan. 11— Gregory (Pappy) Boyington, the Marine flying ace of World War II who commanded the famous Black Sheep Squadron, shot down 28 Japanese planes and won the Medal of Honor, died here this morning at a hospice for cancer patients. He was 75 years old.

Mr. Boyington, a native of Coeur d'Alene, Idaho, was a hero even before the United States entered World War II. As a Marine first lieutenant in 1941, he was persuaded to resign from the service and join the Flying Tigers, Gen. Claire Chennault's American vounteer group in China. While with that unit, he was credited with shooting down six Japanese planes. Ended War as a Prisoner

After the attack on Pearl Harbor had brought the United States into the war, Mr. Boyington rejoined the Marine Corps, was soon promoted to major and, despite surgeons' prediction that a broken leg would end his combat flying, later molded a group of pilots rejected by other squadrons into the Black Sheep, a crack unit that operated in the central Solomon Islands in 1943-44.

Major Boyington spent the war's final year and a half as a captive of the Japanese, after his plane, riddled by bullets, crashed in Rabaul harbor on the Pacific island of New Britain in January 1944. The day he was captured, he was credited with his 26th confirmed kill, more than any other American aviator except Capt. Eddie Rickenbacker, in World War I. Major Boyington's record was later adjusted to 28 planes shot down, a total that was exceeded by other American fliers as the war went on.

His Medal of Honor citation described him as a ''superb airman and determined fighter against overwhelming odds.'' He was also awarded the Navy Cross. Honored in New York

On Oct. 9, 1945, the combat ace, by then a lieutenant colonel, was among 13 Navy and Marine Corps heroes who shared acclaim with Adm. Chester W. Nimitz as an estimated four million New Yorkers turned out to salute the victory in the Pacific.

When he arrived home in Seattle the month before, just shortly after his release from a prisoner-of war-camp, he was greeted with what was described as the loudest ovation ever accorded in that city.

His retirement with combat injuries was followed by a number of years in which he fell victim to alcoholism, a problem he later described candidly and at length in ''Baa Baa Black Sheep,'' a best seller he wrote in 1958 about his exploits with the Flying Tigers and the Black Sheep. ''Baa Baa Black Sheep'' became the basis of a television series starring Robert Conrad.

He was to say in 1956 that the medal drove him to alcohol.

''I was no angel when I got out,'' he said. ''Sure I drank. They made something special of me. There were lots of parties, tours and dinners. They used me for publicity.''

He told then of being helped by an oranization dedicated to helping alcoholics. ''I'm not sensitive about that medal as before,'' he said, ''but I still wish I had never gotten it.'' 'This Stuff Is All Gone'

Of his wartime achievements, Mr. Boyington said in a 1972 interview with The Associated Press: ''This stuff is all gone, and I'd just as soon let it go and forget it. I rarely ever talk about it unless someone brings it up. I don't want to bore anybody or give the impression of being a bore.''

But in the following years he appeared at numerous air shows and other events, promoting his book and talking about his exploits.

He had been treated for cancer several times in the last two decades and moved here in 1971 so that he could undergo cancer treatment at the local Veterans Administration hospital.

Mr. Boyington married Josephine Wilson Moseman of Fresno, who survives him, in 1978. The marriage was his fourth.

Photo of Gregory Boyington (AP)