The race was won by Tatyana Tereshchuk of Ukraine with a time of 54.96, but for Gunnell it was an improvement on her performance in Rome last week where she was fifth in 55.52.
At the same meeting Donovan Bailey lost a soggy 100 metres to Nigeria's Davidson Ezinwa. The Canadian world record-holder finished in 10.28sec to Ezinwa's 10.20. The race began moments after a torrential thunderstorm had abated.
Igor Kovac of Slovakia took the 110m hurdles in a photo-finish with Britain's Tony Jarrett. Both were clocked at 13:45. In another tight finish, Kenya's David Kiptoo won the men's 800m in 1min 44.57sec, just 0.05sec ahead of Hendrik Moganyetsi of South Africa. Vincent Malakwen of Kenya was third in 1:44.75.
The world champion, Jan Zelezny of the Czech Republic, won the javelin with one of the season's best throws of 86.92m; the double Olympic champion Svetlana Masterkova of Russia won the women's 800m in 1:59.29; and Russia's Irina Biryukova took the women's 1500m in 4:05.70 on the flooded track.
The women's 400m was a competition between two Nigerians and six Russians - who finished in that order. Bisa Afoladi of Nigeria won in 50.55sec, followed by her compatriot Falilat Ogunkoya in 50.79. The fastest Russian was Olga Kotlyarova at 51.40.