Dr Gray’s father David died after Dr Ubani gave him an overdose of diamorphine on his first UK out-of-hours shift.
Dr Gray and his brother Rory have sought to raise the profile of the case by speaking out both in the UK and Germany.
But lawyers acting for Dr Ubani have now launched a legal bid in Germany, to stop Dr Gray and his brother Rory from talking to the press. A hearing is due to take place on Monday 9 August.
Dr Gray told GP: ‘My brother and our lawyer will be at the hearing. We are vigorously defending ourselves against this.
‘They are trying to prevent my brother and I talking to the media, even in the UK. The first we heard about this was two weeks ago.
‘We had a letter from Dr Ubani’s lawyer asking us to sign a paper saying we would not go to the press, to pay his lawyer costs and pay a €25,000 fee if we breach the injunction.’
Dr Ubani is also seeking a ruling to block the brothers from coming within 200 metres of him. The pair travelled to a German medical conference earlier this year to confront him and distribute leaflets about the coroner's verdict on how their father died.
Dr Ubani cannot now practise in the UK after being struck off by the GMC. He has been able to continue working in Germany, although he faces a fitness-to-practise hearing on 18 August.
Dr Gray said he believed it was in the public interest for the facts about how his father died, and Dr Ubani’s role to continue to be widely publicised.