Zillah Smith Billany was born at Kingston upon Hull, Yorkshire, England, on 29 May 1859. Her parents were Charlotte Ann Clevelin and her husband, Neiles Boynton Billany, a shipwright. Nothing is known of Zillah Billany's early life and education. She married Edward Keimig, a bookkeeper, probably during the 1880s in Baltimore, Maryland, USA. Keimig had arrived in the United States from Germany as an infant. He died at Baltimore on 28 February 1888. On 3 January 1891 Zillah married Christopher John Gill, a journeyman joiner, at the registry office in Hull. A son was born at Hull in 1894.
In June 1904 the Gill family, along with two Billany relatives, sailed for Wellington, New Zealand. By the end of that year the Gills were living in Palmerston North. Zillah had been a member of a St John Ambulance Brigade nursing division in Hull and passed her first aid and nursing medallion examination in 1898. In Palmerston North she immediately joined the newly formed Palmerston North Nursing Division of St John, which nursed in their own homes people who could afford no other medical care. From 1909 she was secretary of the nursing division and from 1912 to 1930 superintendent. Gill headed the first aid instruction section, teaching classes of up to 30 girls. In 1922 she was made a serving sister of the Order of St John in New Zealand.
In 1918 she was elected to the national executive of the New Zealand Labour Party, and became the first woman to seek a seat on the Palmerston North Borough Council, standing unsuccessfully at a by-election. Gill always thought of herself as representing the working classes. She wanted equal access to health care; supported free hospital treatment for all, and free and compulsory dental treatment in schools; and sought the abolition of the system whereby only doctors selected by a committee of the profession were on the hospital's medical staff.
Zillah Gill also resolutely crusaded for the establishment of a St Helens Hospital in Palmerston North. The hospital board favoured erecting a small maternity ward, chiefly for unwed mothers and women who could afford no other maternity care. Gill vigorously advocated a home that would provide good care for all working-class women. Furthermore, she wanted it established away from the hospital: women connected this institution with suffering and death and feared to go there. She also maintained that there was danger of cross-infection, particularly if the ward were sited in an existing building.
In 1918 a petition supporting these views was signed by over 1,100 local women and presented to both the minister and the Palmerston North hospital board. Gill was one of the instigators of the petition and a member of the deputation that presented it. The board unsuccessfully urged the minister to open a maternity home in Palmerston North. During the next two years the board persuaded the minister to establish a St Helens Hospital in Palmerston North; they themselves would help to finance it. By 1921, when Gill retired from the hospital board, a major fund-raising campaign had been launched, with Gill serving as an executive member of the organising committee. However, insufficient money was raised and in 1926 the board made finance available for building a public maternity home in the hospital grounds.
Zillah Gill's participation in various community activities also encompassed serving on the College Street School Committee from 1917 to 1921, and being a foundation committee member of the Palmerston North sub-branch of the Royal New Zealand Society for the Health of Women and Children (Plunket Society) in 1918. She died on 17 August 1937, at Palmerston North Hospital, survived by her son; Christopher Gill had died in 1930. During many years of public service, Gill had devoted herself to providing community services, particularly for those who most needed them.