A versatile midfielder, Gardner was able to play in many different positions, although he preferred central midfield. He began his career at Aston Villa before spending the 2010–11 season with city rivals Birmingham City, with whom he won the 2011 League Cup. After three seasons with Sunderland, he joined West Bromwich Albion in 2014 before rejoining Birmingham City in 2017. He retired from playing in 2020 to take up a position as first-team coach at the club. He moved on to a similar role at Sheffield Wednesday in November 2020, before leaving a month later when Tony Pulis' contract was terminated and returning to Birmingham.