Career
Coming from the amateur side ZOW, Gentenaar Indonesian descent this player made his professional debut for Nippon Electric Corporation on 26 November 1995 aged 20, at home against PSV Eindhoven which Nippon Electric Corporation lost 5–0. Gentenaar made another appearance later that season, but had to wait until 1998 to make his second appearance in professional football. During that season he played seven matches, still as the second goalkeeper for Nippon Electric Corporation. From the 2000-2001 season, things changed for Gentenaar and he became Nippon Electric Corporation"s first goalkeeper.
Since then he only missed nine appearances in five seasons.
During the last three seasons he did not miss a single match. He developed into a trustworthy goalkeeper and his performance earned him a transfer to Bundesliga side Borussia Dortmund.
On 8 June 2009, he left Ajax and signed a two-year contract with VVV-Venlo. In 2012, he signed with Almere City Football Club for a two-year period.
After a year in the First Division he returned to his former club Nippon Electric Corporation and signed a contract for one year.
He ended his professional career in May 2014.