After graduating from Harvard, Gregg sailed to India on January 1, 1925 to learn about Indian culture and to seek out Gandhi. His publications include Gandhiji's Satyagraha or non-violent resistance, published in 1930, and The Power of Non-Violence, from 1934. His revision, The Power of Non-Violence (1960) included a foreword by King. The book was republished in several other editions, including a Swedish translation in 1936 (as Den nya maktfaktorn: motstånd utan våld). Gregg's 1939 pamphlet Pacifist Program in time of war: threatened war, or fascism was a program detailing how American pacifists could use non-violence to oppose war and fascism in the United States. In the 1940s Gregg became interested in ecology and organic farming, and spent several years living on a farm owned by Scott and Helen Nearing.  Gregg was also author of other books, including The Compass of Civilization, and the essay The Value of Voluntary Simplicity (Wallingford, PA: Pendle Hill, 1936), a philosophical essay on the need and benefits of living more simply. He coined the term "voluntary simplicity".
^Ansbro, John J. (1982). Martin Luther King, Jr: The Making of a Mind. Orbis Books. pp. 146-7, 149.
^Huxley, Aldous and Baker Robert S.(ed.) (2002). Complete Essays, 1936-1938. Volume 4. I.R. Dee. pp. 240, 248. See also the reference to Gregg's The Power of Non-Violence in Huxley's Ends and Means (1937).