Adler was born in Hungary on 15 September 1919 and was recruited by SOE on 21 September 1942. After infiltrating the Cagliari area of Sardinia, Italy by submarine he was captured soon after reaching the shore. The Italian Military Intelligence Service (Survizio per le Informazioni e la Sicurezza) attempted to playback his wireless to London but the incorrect codes were sent, and London was aware it was an attempted wireless deception.
In May 1943 Adler was transferred to the Regina Coeli Prison in Rome that was taken over by the German SD (Sicherheitsdienst) following the Italian surrender. After being interrogated by the SD he gave away no important information and convinced his interrogators he was a British officer named John Armstrong. After the war it was discovered that a ‘John Armstrong’ was in a prison at Lake Bracciano approximately 35 miles from Rome and after the liberation of Rome by Allied forces in June 1944 an investigation attempted to find out what happened to Gabriel Adler aka John Armstrong.
After extensive enquiries it was discovered that on 3 June 1944 Adler was one of 80 prisoners selected by the SD to be transferred to a prison in northern Italy ahead of the Allied advance. The group of prisoners had been assembled in the courtyard at Regina Prison with their hands tied behind their backs. They were put into the backs of lorries and told they were being taken to the SD headquarters at the Via Tasso in Rome, now the home of the Museum of Liberation, and then onto Florence. At 0:30 hrs on 4 June a second group of prisoners were assembled in a similar manner as the first group and put into the back of lorries.
It was later claimed a few of the prisoners believed they were going to be killed and after leaving the compound attempted to escape and though shots were fired a few managed to get away. Another report states the first batch of prisoners also attempted to escape after their convoy of lorries were attacked by Allied aircraft but there was no mention of Adler. According to a priest at the Santo Spirito Hospital in Rome a large number of bodies arrived, and one was listed as an unknown English Soldier, but Investigators concluded Adler was not among the bodies because he was known to be wearing a battle dress over which was a pair of Skiing trousers and a wind jacket and none of the bodies were wearing such clothing.
When the Allies reached Rome the Regina Coeli Prison was empty and a witness said even after the last German lorry was leaving the remaining prisoners were being released. There is a plethora of confusing and contradicting information and the investigating team concluded Adler was most probably dead and was eventually listed presumed dead with no known grave.