Gerasch was fifteen years old when she participated in the 1983 European Championships and placed second behind Ute Geweniger in the 100 m and 200 m breaststroke.

Due to the boycotted the 1984 Summer Olympics in Los Angeles, she participated in Friendship Games, in which she won the gold medal and set a world record for 1 minute 08.29 seconds in 100 m breaststroke.

[3] At the 1986 World Aquatics Championships, she won the 100 m breaststroke and the 4×100 m medley relay, together with teammates Kristin Otto, Kathrin Zimmermann and Kornelia Gressler.

[4][5] In October 1986, she was awarded a Star of People's Friendship in gold (second class) for her sporting success.

[8] Following a test at the European Sprint Swimming Championships 1993 in Gateshead[9] in November 1993,[10] 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.[11] The German Swimming Association shortened the suspension for national competitions.