Gerischer studied chemistry at the University of Leipzig between 1937 and 1944 with a two-year interruption because of military service.  In 1942, he was expelled from the German army because his mother was born Jewish; he was thus found “undeserving to have a part in the great victories of the German Army.”  The war years were difficult for Gerischer, and his mother committed suicide on the eve of her 65th birthday in 1943.  His only sister, Ruth (born in 1913), lived underground after escaping from a Gestapo prison and was subsequently killed in an air raid in 1944.
In Leipzig, Gerischer joined the group of Karl-Friedrich Bonhöffer, a member of a distinguished family, whose members were persecuted and murdered because of their opposition to Nazi ideology.  Bonhöffer descended from an illustrious chemical lineage of Wilhelm Ostwald (1853–1932) and Walther Hermann Nernst (1864–1941). He kindled Gerischer’s interest in electrochemistry, supervising his doctoral work on oscillating reactions on electrode surfaces.  Gerischer completed his doctoral thesis in 1946. Gerischer followed Bonhöffer to Berlin where his Ph.D. supervisor had accepted the directorship of the Institute of Physical Chemistry at the Humboldt University of Berlin. There he also became the department head at the Kaiser Wilhelm Institute for Physical Chemistry in Berlin-Dahlem (later the Fritz Haber Institute of the Max Planck Society).  Gerischer himself was appointed as an “Assistänt”; in 1970 he would return to the Fritz Haber Institute as its Director.  With the Berlin Blockade and the prevailing economic conditions, the post-war research was carried out under extremely difficult conditions.
Gerischer met his future wife, Renate Gersdorf, at the University of Leipzig where she was doing her diploma work with Conrad Weygand.  They were married in Berlin in October 1948.  In 1949 Gerischer moved his young family to Göttingen to join Bonhöffer as a research associate at the newly established Max Planck Institute for Physical Chemistry. In Berlin and Göttingen and especially during the period from 1949 to 1955, Gerischer was interested in electrode kinetics and developed instruments and techniques for their study. It was he who developed the electronic potentiostat, the most widely used instrument of electrochemists. He also monitored fast electrode processes by double potential step and AC modulation. This work laid the foundation for a mechanistic interpretation of electrode reactions and had a lasting impact on our understanding of electrode kinetics. It was recognized by the newly minted Bodenstein Prize of the Deutsche Bunsen-Gesellschaft, which Gerischer and Klaus Vetter jointly received in 1953.
Gerischer was appointed in 1954 to the position of Department Head and Senior Research Fellow at the Max Planck Institute for Metal Research in Stuttgart.  A year later, he received the Habilitation from the University of Stuttgart for his comprehensive study of the discharge of metal ions in corrosion. The years 1954–1961 in Stuttgart were prolific and it was here that Gerischer began his work on semiconductor electrochemistry.  It began with a short note on the electrochemistry of n-type and p-type germanium; a study that grew out of a seminar on solid state physics at the university, where the recent results of Brattain and Garrett on germanium were discussed. Gerischer recognized the theoretical implications of semiconductor electrochemistry in charge transfer and its potential applications in photochemistry and photovoltaic devices. His papers considered the differentiation between Faradaic reactions of electrons and holes (1959), the theory of electron tunneling at semiconductor-electrolyte interfaces, solution Fermi levels, and densities of states.  He extended his studies to metal electrodes which he studied with his electronic potentiostat (1957), to stress corrosion (1957), to hydrogen evolution and hydrogen adatom formation (1957), to fast electrode processes (1960) and to the reaction kinetics of water dissociation, which he probed by the microwave pulse method (1961).
His work was recognized by his appointment as Associate Professor (“Extraordinariat”) in Electrochemistry at the Technical University Munich in 1962–63 followed by his promotion to full professor in 1964 and his appointment as Director of the Institute of Physical Chemistry and Electrochemistry. The 1964–1968 period witnessed a flurry of studies from his group on photoelectrochemistry and photosensitization on electrode materials such as ZnO, CdS, GaAs, silver halides, anthracene, and perylene. In 1969–1970 he was named Dean of Natural Sciences at the Technical University Munich.  Gerischer returned to Berlin in 1970 to assume the directorship of the Fritz Haber Institute of the Max Planck Society, where he continued his studies of electrode kinetics, semiconductor electrochemistry, and photoelectrochemistry. After becoming Emeritus Director of the Institute, he worked with Adam Heller in 1990–1991 at the University of Texas at Austin on the rate-controlling role of adsorbed oxygen in titania-assisted photocatalytic processes.
His honors and awards included the Olin Palladium Award of the Electrochemical Society (1977), Centenary Lectureship, the Chemical Society, London (1979), DECHEMA Medal, Frankfurt (1982), Electrochemistry Group Medal, The Royal Society of Chemistry, London (1987), Galvani Medal, The Italian Chemical Society (1988), and the Bruno Breyer Medal, The Royal Australian Chemistry Institute (1992).