Gerasch was sixteen years old when she participated in the 1983 European Championships and placed second behind Ute Geweniger in the 100 m and 200 m breaststroke. In the subsequent European championships she won her first international title in the 100 m breaststroke.
Her first and only Olympic appearance was at the 2000 Sydney Olympics, where she competed for a unified Germany and finished eighth in the 100 m breaststroke; she was also part of the fourth-place German 4×100 m medley relay team.
Dieter Lindemann was accused of giving Gerasch anabolics when she was thirteen years old. Gerasch claimed that Lindemann gave her pink tablets which she did not take. She claimed that some competitors put the tablets in the aquarium, and that the female fish became more colourful like the male ones.
Following a test at the European Sprint Swimming Championships 1993 in Gateshead in November 1993, Gerasch was suspended in January 1994 for two years for having 16 mg of caffeine, equivalent to six cups of coffee, in her blood compared to the permitted limit of 12 mg. The German Swimming Association shortened the suspension for national competitions. In November 1995, the compulsory ban for caffeine was reduced from two years to three months, and in Autumn 2003, caffeine was removed from the list of banned substances.