The court’s decision has also been condemned by a German medical organisation.
Dr Ubani was struck off by the GMC after administering a fatal overdose of diamorphine to Mr Gray’s father David in 2008, and cannot work in the UK.
But a regional administrative court in Germany has imposed a fine of €7,000 (£6,171) on Dr Ubani, and chose not to stop him working as a doctor.
The Westfalen-Lippe Doctors’ Chamber in the region where Dr Ubani is registered joined Mr Gray in condemning the decision.
A spokesman for the chamber said: ‘The case casts a shadow on health authorities and the reputation of medical doctors in general.
‘We would have preferred our medical association to hold the fitness-to-practise hearing but we were prohibited by the vocational court for administrative affairs of our state.’
Mr Gray, whose brother Stuart is a GP in Worcestershire, said the decision to allow Dr Ubani to continue working was incomprehensible.
‘The medical authorities in both countries say Dr Ubani is dangerous and not competent,’ Mr Gray said. ‘But one local town councillor has the power to decide not to strike him off. I don’t understand why non-medically trained people are taking these decisions.’
In December last year, GP reported that German authorities had been unable to confirm whether Dr Ubani had ever passed medical exams.
Mr Gray warned that current EU rules allowing free movement of doctors between member states could allow Dr Ubani to work outside Germany again.
A House of Lords committee is currently conducting an inquiry into movement of health professionals around the EU.
The GMC is close to finalising a language testing scheme that will tighten requirements on EU doctors.