He entered the Foreign Office in 1933 and served in the British Embassy in Bangkok, then undertook a second tour of duty in Bangkok 1938-41, being captured and interned in Bangkok from April 1941 finaly released in December 1941.

He worked at the Ministry of Information in 1942 then became Second to the Minister in Charge of the Consulate General in Rabat, Morocco in 1942-3. Then began working for "Force 136" Special Operations Executive in India after arriving  in Calcutta in August 1944.

Gilchrist became an intelligence officer of Force 136's Siam Section, where he played significant part in clandestine contacts with Luang Pridi, the anti-Japanese Finance Minister of the Bangkok government.

He was employed organising the undercover resistance force in Thailand and was involved in the repatriation of prisoners of war and humanitarian aid to Japanese indentured (slave) labour from Malaya, Sumatra, Java and Singapore.

Gilchrist is highly critical of Churchill's handling of the situation in Singapore before the Japanese invasion, pointing out that "he was more concerned with the situation in Europe than in the Far east" As he (Churchill) considered that "so long as the Americans had a presence in that part of the World, I have a valuable ally to rely on"  What he did not foresee was the total destruction of the American Fleet in Pearl Harbour

Among his other publications was "Bangkok Top Secret" an account of his work with the Special Operations Force 136.