Gan Choudhury joined the legal profession as an Advocate of Calcutta High Court on March 14, 1975. In connection with his practice in the fields of constitution and taxation, he travels the length and breadth of the country. He was associated for some time with a Calcutta University affiliated law as a guest lecturer. He also served as the Adjunct Professor of Law at the Vinod Gupta School of Management, IIT, Kharagpur, West Bengal.
Gan Choudhury has authored a number of legal treatises. His book Penumbra of Natural justice has drawn high appreciation from legal luminaries like former Chief Justice of India Justice P.N. Bhagwati, former Chief Justice of England Rt. Hon'ble Lord Bingham of Corn Hill, former Chief Justice of South Africa Justice I. Mahomed, former Chief Justice of Australia Justice Jeffrey Miles, former Chief Justice of Sri Lanka Justice G.P.S. DeSilva, and Justice Badrul Haider Choudhury, former Chief Justice of Bangladesh. He has co-authored the book judicial Reflections of justice Bhagwati which has been treated as a reference book by several universities in India as also in foreign countries. He is associated with a number of social and cultural organizations of national repute in various capacities and is the Chairman, Board of Trustees of Indian National Forum of Art and Culture, a premier organization in India in promoting art and culture and helping the needy and deserving artists.