The 65-year-old was found dead in the garden of his gated home in South Weald in June 2015.
Although Essex Police originally thought he died of natural causes, it emerged he had been shot six times.
Greater Essex Coroner Caroline Beasley-Murray said: "I am sure that Mr Palmer was unlawfully killed."
Detectives originally thought he died of natural causes having previously had keyhole surgery to his chest, but it later emerged he had been murdered with the crime having "all the hallmarks" of a contract killing.
Mr Palmer, once described as Britain's richest criminal and thought to have been worth Â£300m, got his nickname after he was arrested on suspicion of melting down gold from the Brink's-Mat robbery, although he was never convicted.
The robbery in 1983 took place at Heathrow Airport when Â£25m worth of gold was stolen. At the time is was the biggest robbery to have ever taken place in the UK.
Mr Palmer, who was one of seven children, was said to have melted the metal down in the back garden of his mansion at Landsdown near Bath.
However, he denied knowing it was stolen and was acquitted in 1987, blowing kisses to the jury from the dock when the verdict was delivered.
Worth around Â£500m in today's prices, most of the gold has never been recovered and the case remains open.
Det Ch Insp Stephen Jennings said the weapon used to kill Mr Palmer has never been recovered.
He told the coroner he believed it was a "contract-style" killing and it was "more than likely" a silenced .32 calibre pistol was used in the murder.
He said the killing could have taken weeks if not months to plan and detectives believe Mr Palmer was being spied on through a hole in his fence.
One line of inquiry is that Mr Palmer was poised to stand trial in Madrid accused of a real estate fraud.
Mr Palmer was burning documents in his garden when he was killed, Det Ch Insp Jennings said.
The coroner expressed her condolences to Mr Palmer's family.
Notorious criminal John "Goldfinger" Palmer was shot six times in a "contract-style" killing, an inquest has heard.