Gunn's success with the Carters and Driver's Union led to his election as President of the
United Trades and Labour Council of South Australia in 1911 and, as his star continued to rise, he was elected to
Adelaide City Council in 1914, serving to 1916, when he became Federal President of the Federated Carters and Driver's Union. "Greatly respected for his tactful courtesy and self-control", Gunn entered the
South Australian House of Assembly at the
1915 election as a Labor member for the
Electoral district of Adelaide. He soon emerged as a leading anti-conscriptionist in the bitter internal Labor fight over
conscription which led to a split in the party. Resigning his seat, Gunn stood unsuccessfully for the federal
House of Representatives Division of Boothby as an anti-conscriptionist candidate at the
1917 federal election but returned to state parliament by regaining his Adelaide seat at the
1918 state election. While many Labor party members were opposed to conscription, party leadership, including premier
Crawford Vaughan, were in favour and either resigned or were expelled from the party, leaving Labor with a rump of inexperienced members of parliament following their
1918 election loss. Gunn, aged 32, was elected leader of the Labor Party and Leader of the Opposition, a move that concerned the Establishment, who recalled the 1910 Driver's Strike and thought he should not be allowed near the reins of power. A media campaign against Gunn began in earnest. A "Thousand Homes Scheme" was carried out to improve the quality and supply of housing for needy families, spending on education was increased, provided better medical facilities and new junior technical schools, higher wages for teachers, and more scholarships. Working conditions were also improved, while measures were undertaken to improve conditions in rural areas "with a visionary afforestation programme in addition to better roads and improvements in access to bore water and technical expertise in agriculture." His reforms impressed many and even received grudging praise from the previously hostile media, who realised he had mellowed into a more moderate figure than the strike leader 15 years previously. Gunn still raised the ire of conservatives through his plans to redraw the Assembly electorate boundaries, introduce a
proportional representation electoral system, introduce adult franchise to the
Legislative Council and establish a state government insurance commission, all of which were defeated by the conservative controlled Legislative Council. Having gained a reputation as a competent administrator and Premier, Gunn surprised many when he suddenly resigned from the Premiership and parliament on 9 August 1926 to accept a Melbourne-based position with the Commonwealth Development and Migration Commission on a greatly improved salary.
Lionel Hill took over the Labor leadership and Premiership. When the position ended, he accepted a federal government appointment in 1930 and moved to
Canberra. When Australian
Prime Minister Joseph Lyons declined to renew Gunn's contract in 1935, Gunn, estranged from his wife and family and possibly in debt, suffered a nervous breakdown. Little was known of his subsequent life. ==Death==