Angelo emigrated to the United States from Palermo, Sicily, Italy and ran a barbershop on the Lower East Side of Manhattan.

When Gaggi turned 17 in 1942, he attempted to join the United States Army, but was rejected due to myopia.

This "no show" job also allowed him to report legitimate, taxable income to the IRS and avoid prosecution for tax evasion.

In October, Gambino boss Albert Anastasia was shot to death in a barber's chair at a Manhattan hotel.

He appointed caporegime Aniello Dellacroce, an Anastasia loyalist, as underboss and gave him control over the Manhattan faction of the family.

By the mid-1960s, Gaggi had established a large clientele of loan shark customers and was also a silent partner in several businesses.

To increase his earnings, he partnered with mobster Roy DeMeo, who was running a stolen car ring in the Brooklyn neighborhoods of Flatlands and Canarsie.

[citation needed] Gaggi and DeMeo began making co-loans to loan shark customers.

[citation needed] However, they were interrupted by a construction crew outside the room that was repairing a faulty air conditioning unit, causing them to flee.

Before his death, he had designated Paul Castellano, his brother-in-law and head of the family's Brooklyn faction, as the new boss.

At a leadership meeting held at Gaggi's house, it was agreed that Castellano would become the new Gambino boss while Dellacroce was retained as underboss.

During this period, DeMeo successfully formed an alliance between the Gambino family and the Westies, a gang of Irish-American criminals that dominated Hell's Kitchen.

[6] By 1979, DeMeo was involved in loan sharking, murder-for-hire, and the operation of an auto theft ring that shipped cars to the Middle East.

Gaggi received a large percentage of profits from these rackets, along with money from DeMeo's drug trafficking.

Gambino capo James Eppolito told Castellano that Gaggi and DeMeo were trafficking drugs.

Eppolito asked for permission to murder Gaggi and DeMeo, but Castellano broke his own rules and sided with them.

However, a witness alerted an off-duty policeman, who soon found Gaggi walking away from the crime scene (DeMeo had gone in a different direction).

After Gaggi's release, Montiglio had become a drug addict and fled New York for fear of punishment from the Gambino family.

Shortly after DeMeo's murder, Montiglio returned to New York to collect an old loan shark debt and was arrested.

To avoid prosecution, Montiglio started cooperating with the government, providing information on Gaggi and the DeMeo crew.

In December 1985, midway through the trial, Castellano was shot to death at the Sparks Steak House in Manhattan on orders from capo John Gotti.

In March 1986, Gaggi was convicted of conspiracy to sell stolen cars, and was sentenced to five years in Lewisburg Federal Penitentiary.

Montiglio provided writers Jerry Capeci and Gene Mustaine information on Gaggi and the DeMeo crew for their book Murder Machine.

In both the book and the television documentaries, Montiglio blamed his criminal actions on Gaggi's bad influence.

In the movie The Iceman, a fictionalized version of Gaggi, named Leo Marks (played by Robert Davi), is a high-ranking member in the Gambino crime family and is killed by Richard Kuklinski.