Hemming reached the rank of sergeant and received an honorable discharge. in October 1958. The following year travelled to Cuba where he gave help to Fidel Castro and his revolutionary forces. In January 1959 he claims he met Lee Harvey Oswald.
A confidential U.S. Army report from March 1960 reports that Hemming was "stationed with Cuban rebel air force in Pinar del Rio. Claims he is a T-33 jet pilot with mission to intercept U.S.-based planes which fly over Cuba bent on destroying cane fields. Was formerly stationed in Isle of Pines. Subject wears Army fatigues, is armed with a pistol, and wears a U.S. paratrooper badge. He states he has been in Cuba for two years. He wears no insignia of rank.''
In 1961 Hemming established Interpen. Other members included Loran Hall, Roy Hargraves, William Seymour, Lawrence Howard, Steve Wilson, Howard K. Davis, Edwin Collins, James Arthur Lewis, Dennis Harber, Dick Whatley, Bill Dempsey, Ramigo Arce, Ronald Augustinovich, Joe Garman, Edmund Kolby, Ralph Schlafter, Manuel Aguilar and Oscar Del Pinto.
Declassified FBI files show that the agency had an informer within Interpen. His code name was MM T-1. In one document dated 16th June, 1961, it said that MM T-1 had “been connected with Cuban revolutionary activities for the past three years”. One document dated 12th May, 1961, claims that Allen Lushane of Miami “had made a trip to Texas to recruit Americans for some future military action against the Government of Cuba”. The document adds that the “first training camp was established by Gerald Patrick Hemming with Dick Watley and Ed Colby running the camp.” In an interview that he gave to John M. Newman on 6th January, 1995, Hemming claims that the FBI informer was Steve Wilson.
Interpen was involved in training members of the anti-Castro groups funded by the Central Intelligence Agency in Florida in the early 1960s. When the government began to crack down on raids from Florida in 1962, Interpen set up a new training camp in New Orleans. During this period Hemming was arrested by Customs for gun-running, but the charges were later dismissed.
In June 1962, The Miami Herald, reported that Hemming was ultimately forced to close the camp on the north edge of Lake Pontchartrain at the behest of the Cuban Revolutionary Council's Miami headquarters. It was claimed that this had happened under pressure from the CIA. When this work came to an end in 1964 Hemming was employed in the construction industry in Miami.
FBI files show that Gerry Hemming told agents in March 1968 that someone had offered to pay him to kill Martin Luther King.
Gerry Hemming has granted long interviews with several writers working on the assassination of John F. Kennedy. This includes Anthony Summers (The Kennedy Conspiracy), Noel Twyman (Bloody Treason), John M. Newman (Oswald and the CIA), Joan Mellen (A Farewell to Justice) and David Talbot (Brothers: The Hidden History of the Kennedy Years).
Some researchers believe that a combination of Interpen members and anti-Castro Cubans were involved in the assassination of John F. Kennedy. This included Hemming, James Arthur Lewis, Roy Hargraves, Edwin Collins, Steve Wilson, David Morales, Herminio Diaz Garcia, Tony Cuesta, Eugenio Martinez, Virgilio Gonzalez, Felipe Vidal Santiago and William (Rip) Robertson.
By 1976, Hemming established an office in the Jose Marti Building in Miami. He told a reporter with the The Miami Herald that he was ''operations director'' of Parabellum Corporation, a group started in October 1971 by Rolando Masferrer and others for the sale of military armaments to domestic and foreign markets.
Gerry Hemming was arrested on 23rd August, 1976, for the illegal transfer of a silencer, and drug smuggling. It seems that this was the point that he began talking about his past work with the CIA. He told one reporter: "All of a sudden they're accusing me of conspiracy to import marijuana and cocaine. Hey, what about all the other things I've been into for the last 15 years, lets talk about them. Let's talk about the Martin Luther King thing, let's talk about Don Freed, Le Coubre, nigger-killers in bed with the Mafia, the Mafia in bed with the FBI, and the goddamn CIA in bed with all of them. Let's talk about all the people I dirtied up for them over the years."
Hemming was convicted by a Miami jury of conspiracy to import marijuana. In 1978 he was sentenced to six months in prison by U.S. District Judge William M. Hoeveler. Hemming was released on appeal bond and the conviction was later overturned.
In August, 1978, Victor Marchetti published an article about the assassination of John F. Kennedy in the liberty Lobby newspaper, Spotlight. In the article Marchetti argued that the House Special Committee on Assassinations (HSCA) had obtained a 1966 CIA memo that revealed Hemming, E. Howard Hunt and Frank Sturgis had been involved in the plot to kill Kennedy. Marchetti's article also included a story that Marita Lorenz had provided information on this plot. Later that month Joseph Trento and Jacquie Powers wrote a similar story for the Sunday News Journal.
On 14th April, 1980, Gerry Hemming was arrested at the Lantana Airport in Palm Beach County with a plane loaded with 723 pounds of marijuana and a cache of Quaaludes. Hemming claimed he was he was working for the U.S. government in an undercover operation. Hemming told Alan J. Weberman that he was working for Mitchell WerBell and Lucien Conein. Hemming was sentenced to 35 years in prison. State records indicate he served seven years.
The HSCA did not publish this CIA memo linking its agents to the assassination of John F. Kennedy. Hunt now decided to take legal action against the Liberty Lobby and in December, 1981, he was awarded $650,000 in damages. Liberty Lobby appealed to the United States Court of Appeals. It was claimed that Hunt's attorney, Ellis Rubin, had offered a clearly erroneous instruction as to the law of defamation. The three-judge panel agreed and the case was retried. This time Mark Lane defended the Liberty Lobby against Hunt's action.
Lane eventually discovered Marchettis sources. The main source was William Corson. It also emerged that Marchetti had also consulted James Angleton and Alan J. Weberman before publishing the article. As a result of obtaining of getting depositions from David Atlee Phillips, Richard Helms, G. Gordon Liddy, Stansfield Turner and Marita Lorenz, plus a skillful cross-examination by Lane of E. Howard Hunt, the jury decided in January, 1995, that Marchetti had not been guilty of libel when he suggested that John F. Kennedy had been assassinated by people working for the CIA.
Hemming moved from Florida to Fayetteville in North Carolina in the 1990s because he wanted to be near the Veterans Affairs Medical Center.
Gerald Patrick Hemming died in his North Carolina home on 29th January, 2008.