Born in Annecy, son of music compositor Jules Marie Victor Gentil (1864-1940) (1864-1940), Jules Charles Henri Gentil first studied the piano with his mother Anna Mockers (1861-1937), who had been a student of Georges Mathias, then with Santiago Riera (1867–1959) at the Conservatoire de Paris, where he won a first prize in 1916.
Having also worked with Lazare-Lévy he often played with his brother violinist Victor Gentil (1892-1973) and with cellist Gérard Hekking.
Gentil helped train many pianists including Jean Micault, Gail Delente, Pierre Froment, Marie-Catherine Girod, Ramzi Yassa, Seth Carlin, David Lively, Michel Sogny, and also composers such as Alain Weber, André Mathieu, and Alain Bernaud.
According to his students' accounts, his teaching was turned towards the research of gestures allowing an easy execution, if necessary with unconventional fingerings, giving importance to the role of shoulders and arms, by bringing a particular care to the pedal.
He did hardly any scale or exercise work, and thus moved away from the habits of his time in French piano teaching.