Gizela (Gisi) Fleischmann grew up in a religious Jewish family. In 1915 she married the Jewish businessman Josef Fleischmann (1886–1942), with whom she had two daughters: Alice (1917–1996) and Judith (1920–1997). In the 1930s she became a leading functionary in the Slovak Zionist women’s movement, as president of the Slovak chapter of the Women’s International Zionist Organization (WIZO). In 1940 Gisi Fleischmann became head of the emigration section at the Jewish Council (Ústredňa Židov) in Bratislava. In 1941 she and other resistance activists founded the underground Pracovná Skupina (Working Group). The group tried to protect Slovak Jews from deportation and help them escape from German-occupied countries. With support from the Jewish Agency, the Working Group managed to bring several hundred Jewish children from Poland to safety in Slovakia.
After the Slovak National Uprising was put down in 1944, Fleischmann was arrested and deported to Sered transit camp. In October 1944 she was taken to Auschwitz-Birkenau extermination camp, where she was murdered.