The retired engineer died less than two hours after Dr Ubani gave him more than ten times the normal dose of diamorphine.
In a scathing report, the NHS regulator, the Care Quality Commission, said Take Care Now under Mr Cocks' leadership "failed to address deep-rooted problems across its entire out-of-hours service," with "tragic consequences."
The company, it said, focused on "business priorities at the expense of clinical services," did not induct Dr Ubani properly, misled NHS trusts and the regulator, and "ignored explicit warnings" about its use of drugs.
Mr Gray's son, Rory, a scientist, said: "I cannot believe what I am hearing. When Cocks' company found out that Ubani had killed my father, they reacted by telling Ubani to leave the country as soon as he could.
"So his own company tried to hide the criminal from the police of which he now is attempting to become a commissioner. This man is obviously unfit to hold any position of responsibility and must not be placed in any position of trust under any circumstances."
Directly elected PCCs, one for each force area outside London, are one of the Government's flagship policing reforms, intended to "sweep away" police bureaucracy and "give people real control" over their force.
The commissioners, paid up to £100,000 a year, will replace unelected police authorities, controlling police budgets and strategy, though "operational matters" will remain in the hands of the local chief constable.
Some analysts have long feared that a low turnout in the November 15 elections could hand "Trojan horse" candidates power and control over policing with only a few thousand votes. The Electoral Reform Society warned last month that the poll could become a farce, with turnout of just 18.5 per cent.
Mr Cocks, the only independent in the Suffolk election, is benefiting from a strong anti-politics tide. Sixty per cent of voters say they do not want party politicians in charge of the police. But his case - and others - show how independents, for all their claims of purity, can have even more baggage than people from the established parties.
In Lincolnshire, an independent candidate, Mervyn Barrett, had to resign last week after his campaign manager told The Sunday Telegraph that he was supported by American right-wing lobbyists and companies backing police privatisation. Mr Barrett has since said that his entire campaign was a confidence trick played on him by the campaign manager.
In Dorset, another independent, Martyn Underhill, is refusing to say which "household name" company and businesses have backed his campaign to the tune of £23,000.
And in Suffolk, Mr Cocks favours further privatisation in the police, though he says it must not affect the front line.
His record of "outsourcing" in the NHS has not deterred him, he says. "We had an unfortunate incident which led to a tragic death. It was, overall, a successful company. We don't accept certain elements of the CQC report, and we never have done.
"[Ubani] was fully qualified in terms of the NHS and the GMC, there was nothing to sugest that he wasn't a proper doctor and able to work as a GP. We didn't know he'd failed an English test."
Mr Cocks defended his performance as chief executive of Take Care Now, saying: "It wasn't until [the Ubani case] got in the papers that everyone started throwing their arms up. I've searched my own conscience. Having discussed it with people who said, there but for the grace of God go us, I don't feel that anything I personally did debars me."
In fact, however, months before Mr Gray's death a senior doctor warned TCN managers that it "was only a matter of time before a patient is killed" if it did not change its procedures on diamorphine.
The drug is not routinely used by German doctors and German TCN locums had already given two non-fatal overdoses to British patients in the months before the tragedy. One of the victims was left brain-damaged and the doctor concerned, a friend of Dr Ubani's, was struck off.
But TCN took no action. Even after Mr Gray's killing, Mr Cocks, according to the CQC, continued to claim that the incident had been a "straightforward error and not a systematic failure."
The regulator slated the company's "flawed" management systems and "complacent" approach to the tragedy, saying TCN "appeared reluctant to accept its shortcomings."
More than half the TCN staff surveyed by the regulator said that "what they would most like to change about TCN was the management."
Mr Cocks' campaign website, however, tells voters: "I relish a challenge! I have had experience with both radio and the county's GPs in developing successful services."
Police sought to extradite Dr Ubani from Germany on charges of manslaughter, but he avoided British justice by accepting a minor conviction and fine for the offence in the German courts. He continues to practice medicine in Germany, even though a court there branded him a "killer" and a "charlatan."
Amid growing controversy about the backers of police and crime commissioner candidates, it can also be revealed that at least one Labour candidate has taken money from a police staff union for his campaigns.
Clive Grunshaw, who is running in Lancashire, declares a £5,000 donation from the Unite union for his campaign. Unite represents thousands of police civilian staff. No other Lab
our candidate declares any financial backing at the Electoral Commission.
Another union, Unison, whose members include PCSos and other police civilian staff and is the second biggest in the police after the Police Federation, has said it is "supporting" the campaigns of three Labour candidates, John Prescott in Humberside, Shaun Wright in South Yorkshire and Mark Burns-Williamson in West Yorkshire. "We are supporting them because it is extremely important we have strong representation at the highest level," said the union's Yorkshire and Humberside regional officer, Chris Jenkinson.
Lord Prescott said last night that the support was only verbal and he had received no money or donations in kind from Unison.
A spokesman for the other two candidates said: "All donations made to the Labour Party are declared to the Electoral Commission in compliance with the law." It appears likely that any union money given to Labour police commissioners will appear in the registers as a donation to the party generally, rather than to them. The Electoral Commission said it was looking into a complaint about donations to Mr Wright.
Sam Chapman, a Conservative councillor and former police officer who runs Top of the Cops, a website extensively covering the elections, said: "It doesn't feel right - you're going to have to make decisions about staff whose representatives have given you money. You might have to decide, for example, about the balance of PCSOs versus police in your force, with the PCSOs in a union which has funded you and the police officers in the Federation, which hasn't."
