Coroner William Morris concluded that Dr Ubani's tiredness and rushed induction led to him administering 100mg of diamorphine instead of 10mg at Mr Gray's Cambridgeshire home in February 2008.
Mr Williams called the shortcomings that led to Dr Ubani being hired by NHS Cambridgeshire as 'a lamentable situation'.
West Yorkshire NHS Central Services Agency refused to let Dr Ubani work after he failed a language test. But Cornwall and Isles of Scilly's primary care support agency, part of the PCT, accepted him onto the performers list without a language test or asking if he had ever failed one. This entitled Dr Ubani to work for any PCT in England.
The inquest heard how a nurse working for TCN on the weekend of Mr Gray's death reported concerns about Dr Ubani, but received no response from the firm.
A busy GP asked to induct Dr Ubani at short notice, wrote to TCN to say the assessment was not thorough enough.
Mr Morris said the EU council directive that allows European GPs to work in the UK without having their competence checked was 'the root of the problem'.
Mr Gray, who suffered from renal colic, told Dr Ubani his usual GP treated his pain with 100mg of pethidine.
Mr Morris said: 'I have little doubt that the mention of 100mg influenced Dr Ubani in the gross error he was to make.' Dr Ubani did not re-examine or check on the patient after the injection.