[1] He was a Labor member of the Northern Territory Legislative Assembly, holding the seat of Fannie Bay in Darwin from the retirement of then Chief Minister Clare Martin at the 2008 election until his resignation in July 2022.

[6] The election saw Gunner retain the seat for the Labor Party by only a slight margin, over Garry Lambert, former alderman and acting Lord Mayor.

After a failed attempt in 2013,[7] on 19 April 2015 Gunner announced he would stand for the leadership of the Northern Territory Labor Party against incumbent Delia Lawrie, who was facing criticism of her conduct during an inquiry into the gifting of heritage property Stella Maris to Unions NT while she was a minister.

[10] He held the portfolios of Major Projects, Northern Australia Development, Police, Fire and Emergency Services, Open and Transparent Government and Business and was Shadow Treasurer.

Gunner declared that Territorians had "rejected the chaos of the last four years", in part a reference to the numerous reshuffles that had taken place during the previous government, including multiple leadership spills.

However, with Labor's victory beyond doubt even though counting continued until early September, Gunner arranged to have Administrator John Hardy swear himself, Natasha Fyles, and Nicole Manison as an interim three-person government until all prospective ministers were confirmed as elected.

[15] Gunner held 38 portfolios in the three-person ministry in contrast to Manison solely as Treasurer and Fyles as just Attorney General and Justice Minister.

He won the third-largest majority government in Territory history (just percentage points behind Labor's 2005 landslide), and faced only two CLP members as opposition − fewer than the five independents in the chamber.

[18] Despite Labor's massive majority, Gunner retained CLP-turned-independent Kezia Purick as Speaker of the Northern Territory Legislative Assembly for a second term.

Labor heavily centered their campaign around Gunner's leadership and government handling of the COVID-19 pandemic, even going as far to say the election was, "literally the difference between life and death,"[26] which drew criticism from his opponents.

[27] Before the election, opinion polls suggested that Labor would suffer an almost certain swing against them, with the newcomer Territory Alliance party posing a serious threat to the typical two-party system that has dominated NT politics.

[28] Nevertheless, on election night, ABC psephologist Antony Green projected that Labor would win at least a minority government almost three hours after the polls closed.

Gunner later declared victory an hour later, saying that he was "very confident" that Labor had won another term, with the Country Liberals refusing to concede defeat on election night.

In October 2021, the United States Senator from Texas Ted Cruz posted a tweet that was critical of the Northern Territory's vaccine mandates.

[35][36] On 10 May 2022, Gunner announced his resignation from his positions as the NT Chief Minister and Treasurer, citing a desire to spend more time with his family.