Gregory was born Fritz Gugenheim in London, but changed his name as a result of the anti-German sentiment during the First World War, which culminated in the destruction of his laboratory notebooks by colleagues at the Cheshunt Experimental Station.
Gregory's intention when applying had been to study chemistry, but after attending a lecture by John Bretland Farmer he switched to botany, gaining his ARCS in 1914 and BSc in 1915, both with first class honours, and also won the Forbes prize.
By 1928 his work was visible and important enough that he was asked to advise the Empire Cotton Growing Corporation on their irrigation techniques in Sudan, setting up statistical studies so helpful that his final report was widely used in the development of agronomy within the country.
After Blackman's retirement in 1937 Gregory was made head of the laboratories, although work at the university was heavily disrupted by World War II.
'[3] Around the same time he was also elected a foreign member of the Indian Society of Plant Physiologists and the United States National Academy of Sciences.