Gerstner remained in Paris till May 1944. (The city was liberated from German occupation a couple of months later.) He was able to support the French Resistance by secretly passing on information and, especially before 1943, by providing travel permits for the unoccupied southern part of France, thereby rescuing numerous Jewish families facing the threat of deportation to death camps in Germany. During the second half of 1944 and then till the war ended in May 1945 Gerstner was back in Berlin employed, at least officially, at the Foreign Ministry, in the "Special office for news procurement" ("Sonderreferat für Nachrichtenbeschaffung"). It has been suggested - perhaps uncharitably - that this gave him an opportunity to hone a peculiar talent for charming people into providing information that he could then pass on for pecuniary gain, which after 1975 would by of significant value to the East Germany Ministry for Security (Stasi). Other sources refer to his having undertaken "illegal political work" - implicitly on behalf of the still illegal Communist Party - in the Berlin quarter of Wilmersdorf where he had ended up. Fellow antifascists with whom he operated included Leo Dyck, Herbert Eppinger, and .