He has previously played for Luleå HF of the Swedish Hockey League (SHL), as well as the Ottawa Senators.

[3] With the 2020–21 season delayed due to the COVID-19 pandemic in North America, Gustavsson was loaned to Södertälje SK of HockeyAllsvenskan.

[9] On 12 July 2022, Gustavsson was traded by the Senators to the Minnesota Wild in exchange for goaltender Cam Talbot.

[9] He started the season as the backup to Marc-André Fleury and got his first NHL shutout on 10 December 2022 in a 3–0 win over the Vancouver Canucks.

He made 51 saves, setting the Wild franchise playoff record, in a double-overtime 3–2 win.

[13] He again split the net with Fleury during the 2023–24 season, starting 43 games for the Wild, but the team failed to make the 2024 Stanley Cup playoffs.

He stopped 29 of 31 shots in a 4–2 victory over the United States to get Sweden into the gold medal game in the tournament.