Gunnell won the 400 m hurdles at the 1992 Olympic Games in Barcelona, running 53.23 to defeat Sandra Farmer-Patrick. She also anchored the British 4 × 400 m quartet to a bronze medal. In 1993, she reached her peak, when she set the world record in the 400 hurdles to win gold in the World Championships in Stuttgart, winning in 52.74, narrowly ahead of Farmer-Patrick who ran 52.79, also inside the old record.