He played for Linköpings HC of the Elitserien (SEL) and the Toronto Maple Leafs and St. Louis Blues of the National Hockey League (NHL).

A native of Sweden, Gunnarsson began his minor ice hockey career with the Örebro HK and Linköpings HC junior teams before being selected in the seventh round, 194th overall, of the 2007 NHL Entry Draft by the Toronto Maple Leafs.

[2] Growing up in Sweden, Gunnarsson began his minor ice hockey career with the Örebro HK and Linköpings HC junior teams.

[1] Gunnarsson was selected in the seventh round, 194th overall, of the 2007 NHL Entry Draft by the Toronto Maple Leafs but chose to continue playing in the Swedish Elite League for Linköpings HC.

He ended the season with 16 points in 53 games and on 2 June 2009, began his North American professional career by signing an entry-level contract with the Maple Leafs.

[5] He attended the Maple Leafs Rookie Camp in September[6] and was assigned to their American Hockey League (AHL) affiliate, the Toronto Marlies, to begin the 2009–10 season.

[8] He made his NHL debut on 14 November in a loss to the Calgary Flames, during which he played more than 21 minutes of ice time and was praised by coach Ron Wilson as looking like the Leafs' best defenceman.

[9] Gunnarsson stayed with the Leafs for the remainder of the season and scored his first NHL goal on 29 January 2010 against Martin Brodeur of the New Jersey Devils.

[16] He returned to the Leafs lineup only to suffer a season-ending separated shoulder injury during the teams' final home game against the Buffalo Sabres on 4 April.

[17] Due to the 2012–13 NHL lockout, Gunnarsson was released by the Leafs in order to playout the season with his hometown team Örebro HK.

At the beginning of the season, during a game against the New York Rangers on 16 October, Gunnarsson, Kyle Brodziak, and Jori Lehtera all sustained lower-body injuries.

He sat as a healthy scratch during the Blues' season opener against the Chicago Blackhawks[34] and missed the next two games due an upper body injury.

[35] On 18 June 2017, the Blues left Gunnarsson unprotected to the 2017 NHL Expansion Draft for the Vegas Golden Knights,[37] but David Perron was chosen instead.

[40] Gunnarsson played 68 games with the Blues during the 2017–18 season before suffering a season-ending ACL tear, forcing him to miss six months in order to recover.

During intermission prior to overtime, Gunnarsson approached coach Craig Berube at the urinal and told him he just needed "one more shot," after hitting the crossbar in the third period.

Gunnarsson made his junior international debut for Sweden at the 2004 IIHF World Under-18 Championships, where he played in six games and recorded 24 penalty minutes.