On graduation, Gy worked as a chemical engineer for CMCF, Congo, before returning to Paris in 1949 as a research engineer for the mining and processing trade organisation Mineraux et Metaux. It was during this time that Gy began to address fundamental issues in sampling, especially those concerned with characterising industrial bulk chemicals, aggregates and chemical processes. Gy became head of the mineral processing laboratories and, by 1962, technical manager, during which time he continued to develop his sampling theory, usually articulated in terms of the seven sampling errors. Since 1963 he has worked as an industrial sampling consultant.

Gy received doctorates in physics (1960) and mathematics (1975) from the University of Nancy but has always worked outside the academic mainstream, publishing 9 books and over 175 papers, including: