stylistic:tone, quality:correction
1947–1949: Career setbacks

In 1947, Peck co-founded The La Jolla Playhouse at his birthplace with Mel Ferrer and Dorothy McGuire. This summer stock company presented productions in the La Jolla High School Auditorium from 1947 until 1964. In 1983, the La Jolla Playhouse re-opened in a new home at the University of California, San Diego, where it operates today. It has attracted Hollywood film stars on hiatus, both as performers and enthusiastic supporters, since its inception.

Peck's next release was the modest-budget, serious adult drama, The Macomber Affair (1947), in which he portrays an African hunting guide assisting a tourist couple. During the trip, the wife, played by Joan Bennett, becomes enamored with Peck, and the husband gets shot. Peck was very active in the development of the film, including recommending director Zoltan Korda. The film received positive reviews but was mostly overlooked by the public upon its release, which Peck would later say disappointed him.

In November 1947, Peck's next film, the landmark Gentleman's Agreement, directed by Elia Kazan, was released and was immediately proclaimed as "Hollywood's first major attack on anti-Semitism". Based on a novel, the film has Peck portraying a New York magazine writer who pretends to be Jewish so he can experience personally the hostility of bigots. It was nominated for eight Academy Awards, including Peck for Best Actor, winning in the Best Film and Best Director categories. It was the second-highest top-grossing film of 1948. Peck would indicate in his later years that the film was one of his proudest works. Upon release, Gentleman's Agreement was widely praised for both its courageousness and its quality, Peck's performance has been described as very convincing by many critics, both upon release and in recent years. In recent decades, critics have expressed differing opinions regarding Peck's portrayal, the quality of the film by modern standard, and the film's effectiveness at addressing anti-semitism, with film writer Matt Bailey writing " Gentleman's Agreement may have been an important film at one time, but was never a good film,"

Peck's next three releases were commercial disappointments. The Paradine Case was his second and last film with Hitchcock. When producer David O. Selznick insisted on casting Peck for the movie, Hitchcock was apprehensive, questioning whether Peck could properly portray an English lawyer. In later years, Peck did not speak fondly of the making of the movie. Released in 1947, The Paradine Case was a British-set courtroom drama about a defense lawyer in love with his client. It had an international cast including Charles Laughton, Ethel Barrymore and Alida Valli as the accused. The movie received positive reviews, with many complimenting Peck's performance, but it was panned by the public, only recouping half of the $4.2 million production costs. In recent decades, the film was criticized by most prominent writers, although critic's praised Peck's acting. Writers Paul Condon and Jim Sangster stated that "Peck is vulnerable yet believable in a role that requires significant delicacy of touch to maintain viewer's loyalty and interest."

Peck shared top billing with Anne Baxter in the Western Yellow Sky (1948), named for the ghost town where Peck's group of bank robbers seek refuge and encounters the spunky tomboy Baxter, her grandfather, and their gold. Peck gradually develops an interest in Baxter's character, who in turn seems to rediscover her femininity and develops an interest in him. Critics who commented on Peck's performance felt it to be solid, but said the plot was slightly unbelievable. The film was only moderately commercially successful. A year later, Peck was paired with Ava Gardner for their first of three films together in The Great Sinner (1949), a period drama-romance where a Russian writer, Peck, becomes addicted to gambling while helping Gardner and her father pay back their debts. Peck ended up becoming great friends with Gardner, and would later declare her his favorite co-star. Their friendship lasted the rest of Gardner's life, and upon her death in 1990, Peck took in both her housekeeper and her dog. The film received unfavorable reviews usually describing it as dull, and the public was not interested, rendering it a commercial disappointment. In modern times, the film has received mixed reviews but TV Guide says "this often gripping film" has strong performances, that "Peck is powerful" in his portrayal. Peck initially rejected the film, his last movie under his MGM contract, eventually agreeing to do it as a favor to the studio's production head.

His second 1949 release, Twelve O'Clock High (1949), was the first of many films in which Peck embodies the brave, effective, yet human, "fighting man". Based on true events, Peck portrays the new commander of a “hard luck” U.S. World War II bomber group tasked with instilling discipline and pride into the pilots and crews. He believes the former commander failed because he identified too closely with the men and his overly protective attitude caused the squadron to fail in its mission. Peck succeeds in whipping the command into shape, but finds himself caring deeply for his men and finally breaks down after losing his adjutant on a particularly rough mission over Schweinfurt. The National Board of Review ranked it in their top ten films of the year and it received four Academy Awards nominations, Best Actor for Peck. Peck was later recognized in the New York Film Critics Circle for the role. Twelve O'Clock High was a commercial success, finishing tenth in the 1950 box office rankings. The film received strong reviews upon release. Recent critics maintain positive opinions. Evaluations of Peck's performance were positive, with The New York Times describing "High and particular praise for Gregory Peck... Peck does an extraordinarily able job in revealing the hardness and the softness of a general exposed to peril." Film historian Peter von Bagh considers Peck's performance "as Brigadier General Frank Savage to be the most enduring of his life".