Gentner received his doctorate in 1930 from the University of Frankfurt. From 1932 to 1935 he had a fellowship which allowed him to do postdoctoral research and study at Curie's Radium Institute at the University of Paris. From 1936 to 1945, he was a staff scientist at the Institute of Physics at the Kaiser Wilhelm Institute for Medical Research, in Heidelberg. One of his areas of specialization was nuclear photoeffects. He was granted his Habilitation from the University of Frankfurt in 1937. At the end of 1938 and early 1939, he visited the Radiation Laboratory at the University of California, Berkeley; upon his return to Germany, he participated in the construction of a cyclotron at Heidelberg. During World War II, he participated in the German nuclear energy project, also called the Uranium Club. After World War II, Gentner became a professor at the University of Freiberg.
In 1956, Gentner was appointed Director of the Synchrocyclotron Department at CERN. In 1958, he became director of the new Max Planck Institute for Nuclear Physics at Heidelberg. From 1967 to 1970, he was chairman of the Physicochemicl-technical Section of the Max Planck Society. From 1969 to 1971, he was President of the Science Policy Committee and President of the Council at CERN. From 1972, he was Vice-president of the Max Planck Society. From 1975, he was a member of the board of governors at the Weizmann Institute of Science, Israel.
Gentner helped found a number of European scientific organizations during the 1960s.
From 1925 to 1930, Gentner studied at the Friedrich-Alexander-Universität Erlangen-Nürnberg and the Johann Wolfgang Goethe-Universität Frankfurt am Main. While in his first semester at Erlangen, his father died, so he returned to Frankfurt to help care for his mother and continued his education at Frankfurt. He received his doctorate in 1930 under Friedrich Dessauer, who was Director of the Institut für die physikalischen Grundlagen der Medizin (Institute for the Physical Fundamentals of Medicine), at the University of Frankfurt. His thesis was on the range of electrons in matter and their biological effects. In 1932, he was an auxiliary aid (Hilfsassistent) to Dessauer. From 1933 to 1935, he was a fellow of the Oswalt-Stiftung (Oswalt Foundation) of the University of Frankfurt and a fellow of the Carnegie Foundation, whose assistance he used to study at the Radium Institute of the University of Paris, which at that time was under the leadership of Marie Curie.