Both of the ex-squaddies were experienced British Army soldiers; Mr Gatley, from Sandbach in Cheshire, served six years with the Third Battalion of the Rifles, while Mr Sibley was a veteran of the Royal Logistic Corps.
They had wanted to use their skills to help people in Ukraine, and separately joined war efforts in the country in mid-March.
Darren Salter, senior coroner in Oxford, formally ruled on Tuesday that they were both “killed while on active service for the Ukrainian foreign legion”.
Dean and Sally, Mr Gatley’s parents, watched the proceedings via videolink. In a statement read out to the court, his mother recounted her son’s passion for the armed forces and said the couple were “very proud of him for everything he had achieved, and devastated that his life has been cut short”.