Before joining
VfB Stuttgart, Gentner played at TSV Beuren and
VfL Kirchheim. In the 2004–05 season, he played 28 times and scored six times for the
Regionalliga (third division)
team of VfB Stuttgart. In the same season, he had his
Bundesliga debut against
Hertha BSC on 20 February 2005 (1–0). He scored his first goal in a
UEFA Cup game against
Domžale on 25 September 2005. In 2006, Gentner's contract in Stuttgart was extended until 2010. On 18 July 2007, he was loaned to
VfL Wolfsburg until summer 2009 and on 11 August 2008 Gentner signed a permanent deal with Wolfsburg. On 8 January 2010, Gentner announced that he would be joining
VfB Stuttgart at the end of the season. After seeing out his contract with the Wolves which ended on 1 July 2010, Gentner returned to Stuttgart on a
free transfer. On 6 January 2013, Gentner extended his contract with VfB Stuttgart until June 2016 with an option for a further year. Following
Serdar Tasci's departure to
Spartak Moscow, it was announced that Gentner is to be his successor as the new club captain. In May 2016 Gentner extended his contract with VfB Stuttgart until June 2019. On 5 July 2019, Gentner signed a contract with club
Union Berlin. In September 2022, Gentner announced that he would retire from playing at the end of 2022. At the same time,
VfB Stuttgart announced that he would join the club on 1 January 2023 as head of the professional player department. ==International career==