Mabel Winifred Baker was born, probably on 1 June 1890, in Moorlinch, Somerset, England, to Jane Reed and her husband, Richard Baker, a farmer. After training as a nurse at Guy's Hospital, London, she emigrated to New Zealand. In January 1920 she began district nursing at Whataroa, near Franz Josef Glacier. South Westland was very isolated, and Mabel Baker provided an important service to the community.
A large and competent woman, Mabel Baker was one of the few women of the time to drive a car. The road ended at Fox Glacier, however, and Mabel rode a horse to attend cases in the far south of the region. These journeys were often hazardous, with numerous rivers to be forded, but she coped well and many admired her spirit. Well-known Franz Josef personality John Schilling wrote: 'Sister Baker's no reneger / Always got a smile / Tends to cases rain or blazes / Would ride a hundred miles'.
Paradoxically, Mabel was frightened of staying alone in her remote farmhouse, and initially a girl from Franz Josef, Jessie Green, stayed with her. On 17 May 1921 she married returned serviceman and farmer Frank Gunn in the first wedding at St Luke's Church, Whataroa. Mabel was married in her nurse's uniform. The couple were to have one child. After her marriage, as well as milking cows and playing a key role in the management of their farm at Te Taho, near Whataroa, Mabel Gunn opened a maternity hospital in an annexe of their house, where she delivered many south Westland babies. She also operated the local telephone exchange.
In late 1948 New Zealand's first woman cabinet minister and minister of health, Mabel Howard, visited Mrs Gunn's maternity home, which had continued to serve as south Westland's maternity hospital. She decided that the region needed a better facility, and at her instigation, a five-bed hospital was built in Whataroa. This opened in March 1953 with Mabel Gunn as its first matron, a position she held for many years.
'Sister Gunn', as most people called her, was a controversial personality. Her English accent and authoritarian manner alienated her in a region where people were renowned for their informality. However, she was respected for her medical judgement. For instance, Ethel Richardson, an artist with a serious heart condition, longed to see the rata trees in the glacier valley, but refrained from going until Gunn authorised the expedition. Others had negative experiences. One woman felt desolate every time her husband left her at the hospital to give birth, saying that Mabel was hard to the point of cruelty in her disciplinary attitude to young mothers.
Gunn's management style at Whataroa Hospital was equally controversial. She made her own rules and was generally regarded as over-protective of her situation. This worried the Westland Hospital Board and one member mounted a public campaign to have her dismissed. He failed. Those who knew her agreed she should have retired years before she did in 1963, but nursing was her life.
Despite her confrontational style, in an essay she wrote for Women of Westland 1860–1960 Mabel Gunn demonstrated a warm appreciation of the West Coast lifestyle and people. She played a crucial role in providing health services over many decades and did much to further the development of hospital services in south Westland. She was appointed an OBE in 1960.
The Gunns eventually retired to Nelson, where Frank was killed in a car accident in 1964; Mabel was injured. She died on 7 January 1970 in Westland Hospital, Hokitika, survived by her son.