Mr. Kaplan conducts the New York Philharmonic in 2008. (Hiroyuki Ito/Getty Images)

Gilbert E. Kaplan, who amassed a fortune on Wall Street that enabled him to fulfill his fantasy of becoming an orchestral conductor, confining himself to a single work, Mahler’s Second Symphony, but stunning a skeptical musical world with his command of the daunting score, died Jan. 1 in New York City. He was 74.

The cause was cancer, said his daughter Emily Kaplan.

Even before Mr. Kaplan set out to conduct Mahler’s “Resurrection” symphony, a feat that has been described as the musical equivalent of scaling Mount Everest, he demonstrated that he was a man of brass.

He was in his mid-20s, just starting out as an economist with the American Stock Exchange, when he borrowed more than $100,000, mostly from Seagram’s liquor baron Gerald Bronfman, to start the financial publication Institutional Investor. Launched in 1967 under the editorship of George J.W. Goodman, later known as the host of “Adam Smith’s Money World” on PBS, it became a go-to guide for money managers and financiers.

“The hottest new publishing company since Hugh Hefner thought of turning bunnies into a staple of American manhood” was how Myron Kandel described it in the Wall Street Letter in 1969.

The Institutional Investor enterprise grew to include conferences and newsletters and made Mr. Kaplan a millionaire many times over, although exactly how many times over was unclear. In the 1980s, he sold the operation to Capital Cities Communications for an undisclosed sum reportedly between $70 million and $100 million.

While building his business, Mr. Kaplan had nurtured another passion — some observers called it an obsession — first privately, then in public, almost spectacular fashion.

The passion dated to one day in 1965, when Mr. Kaplan, still toiling on Wall Street, accompanied a friend to New York City’s Carnegie Hall to hear a rehearsal of Mahler’s Second Symphony by the American Symphony Orchestra, under the venerable conductor Leopold Stokowski.

The composition is regarded as a monument of 19th-century music, a sprawling work of five movements requiring an orchestra of 100, a choir of up to 200 and two soloists. Running as long as 90 minutes in performance, it leads the listener on a transcendent exploration of the meaning of life and death.

“And that was it,” Mr. Kaplan told the Boston Globe in 1989. “Zeus threw the bolt of lightning. I walked out of that hall a different person. . . . There’s been nothing that put me in orbit the way this did.”

Mr. Kaplan dove into an all-consuming study of the composition and woke up on the cusp of his 40th birthday, he said, fixated on the notion that he must conduct it — the only way, he thought to “unlock the mystery.” He floated the idea among friends, who told him it was “insane.”

“They were right, of course,” he told the Globe. “I tried to read a book on conducting and couldn’t understand it, though I consoled myself with the thought that if I tried to read a book on tying a shoelace, I probably couldn’t understand that, either.”

His financial means exceeded perhaps only by his determination, Mr. Kaplan engaged the conductor Charles Zachary Bornstein as his teacher. For a month, the two men worked together, nine hours a day, at Mr. Kaplan’s home in East Hampton, N.Y., studying the intricacies of the work.

Later, Mr. Kaplan hopped around the world attending performances of the symphony and meeting with conductors including Zubin Mehta, James Levine and Georg Solti.

“What a pleasure it is to meet a man from Wall Street with whom I talk about music,” Mr. Kaplan recalled Solti remarking, “because when I meet my colleagues all I talk about is money.”

In 1982, after a series of rehearsals, Mr. Kaplan hired the American Symphony Orchestra to present the Second Symphony under his baton at Lincoln Center. Estimates of the price tag of the affair, an invitation-only celebration of the 15th anniversary of Institutional Investor, reached $100,000.

The orchestra, evidently skeptical of Mr. Kaplan’s abilities, placed two conditions on the performance: no tickets would be sold to the public, and there would be no media reviews. Even Mr. Kaplan seemed to harbor doubts about his preparedness. If the performance derailed, he planned to pivot on the podium and tell his audience, “Ladies and gentlemen, dinner is served.”

Despite the ban on reviews, Leighton Kerner, classical music critic for the Village Voice, was in the audience made up mainly of Wall Street celebrities, along with at least one senator and two prime ministers. He declared the interpretation “one of the five or six most profoundly realized Mahler Seconds” in the previous 25 years. Even those attendees not schooled in music seemed to recognize that they had witnessed something remarkable.

“I had the feeling,” he told the London Evening Standard, “that people in the audience were urging me to fulfill my dream because each of them had a secret ambition. . . . They were up with me that night, playing baseball for the Yankees, writing the book they never wrote, getting the girl they never got.”

Encouraged, perhaps even emboldened, by the evening’s success, Mr. Kaplan went forward with his Mahlerian devotion. In 1984, he purchased from a foundation the original manuscript of the Second Symphony. After rigorously comparing the original with the commonly used version, he claimed to have found 300 errors in the latter and co-edited a new score that won the approval of the International Gustav Mahler Society in Vienna.

As word traveled in the music community of Mr. Kaplan’s achievements, he became a sought-after scholar and conductor of Mahler, although, with few exceptions, he limited his conducting appearances to the Resurrection.

More than 50 orchestras around the world engaged Mr. Kaplan, among them the Vienna Philharmonic, the London Philharmonic, the Moscow Symphony Orchestra, the Israel Philharmonic, the China National Symphony Orchestra, the orchestra of La Scala opera house in Milan and the Los Angeles Philharmonic. Mr. Kaplan’s recording of the Second Symphony with the London Symphony Orchestra was described in news accounts as the best-selling Mahler recording ever.

Mr. Kaplan had his detractors. In 2008, when he conducted the New York Philharmonic in a performance marking the centennial of the Second Symphony’s premiere in the United States, some members of the orchestra complained about his leadership of the work, decrying him as an “impostor” and a “charlatan.” Steve Smith, who reviewed the event for the New York Times, seemed to disagree.

“Every gesture had purpose and impact, and the performance as a whole had an inexorable sweep,” he wrote. “To think there is nothing else to know of Mahler’s Second beyond what Mr. Kaplan has to show would be a mistake. But it seems likely that no one is better equipped to reveal the impact of precisely what Mahler put on the page.”

It was that fidelity to Mahler’s original intent that perhaps conferred on Mr. Kaplan all the authority that he possessed.

“I feel that I’m always working for Mahler,” he told the London Guardian in 2003. “Apart from tempo, I don’t ask the orchestras I conduct to follow my interpretation. I ask them to observe what Mahler wrote, and on most everything I don’t lose an argument because it’s in the score.”

Gilbert Edmund Kaplan was born in New York City on March 3, 1941. A late brother was Joseph Brooks, the jingle writer who also penned the pop hit “You Light Up My Life.”

Mr. Kaplan took piano lessons as a boy but, disinclined to practice, did not seem destined for a life in music. He studied economics at Duke University before graduating from the New School in New York City in 1964.

Survivors include his wife of 45 years, the former Lena Biörck of New York, for whom he purchased the ring that Mahler had once given to his wife, Alma; four children, Kristina Wallison and Claude Davies, both of New York, and John Kaplan and Emily Kaplan, both of Los Angeles; and eight grandchildren.

Mr. Kaplan, who conducted from memory and with a baton owned by Mahler, accepted no payment for his performances because he did not consider himself a professional. Among other confessed weaknesses, he could barely read music apart from the Second Symphony. He was, he said, an amateur “in the best sense of the word.”

“I didn’t set out to do it because I had some grandiose ambition to be a conductor,” he once told an Australian newspaper. “I did it because I wanted to get inside the music. . . . There’s a real explanation of life and death in that music and I wanted to get to the bottom of it.”