John George Gamble was the eldest son of Dr Harpur Gamble of the Royal Navy. From 1854 to 1859 he atended the Royal Navy School, New Cross, and then went to Magdalen College, University of Oxford, where he qualified as Master of Arts (MA) in mathematics in 1864. Two years later he became an engineering pupil of John Hawkshaw. In 1870 Hawkshaw appointed him as resident engineer of the new sewers at Brighton, of which he compiled an account, "The Brighton intercepting and outfall sewers" in the Minutes and Proceedings of the Institution of Civil Engineers for 1875-1876. In 1874 Hawkshaw sent him to Brazil to investigate several of the important harbours of that country. He became an associate (1869) and later a member (1876) of the (British) Institution of Civil Engineers and was elected a Fellow of the Royal Meteorological Society.
Shortly after returning to England in April 1875 Gamble was appointed in September that year as hydraulic engineer of the Cape Colony, a post he held until his retirement on pension in May 1886, when the post was abolished. Upon his arrival he was asked to advise the town council of Cape Town on the best method of increasing the city's water supply and steps required to store the water, and delivered his Report on the water supply of Cape Town before the end of the year. He also made an extended tour of the northern districts of the colony. His work related mainly to the provision of water to the colony's towns and led to several publications. For example, his paper on "A storage reservoir in the Achterveld" was read before the South African Philosophical Society and published in its Transactions (1877-1879, Vol. 1, pp. 219-). A second report on the water supply of Cape Town was issued in 1881 and a description of the waterworks of Port Elizabeth, designed according to his recommendations, in 1883. Other government works executed under his supervision included a storage reservoir at Brandvlei, a reservoir at Vanwyksvlei, and waterworks at King William's Town, East London, Queenstown, Somerset East Riversdale and Graaff Reinet. In 1887 his comprehensive paper on "Water-supply in the Cape Colony" was published by the Institution of Civil Engineers in London.
Most of Gamble's scientific work in the Cape Colony was in the field of meteorology. He served on the Cape of Good Hope Meteorological Committee from 1882 to 1887 and with Prof. F. Guthrie* played an important part in organising simultaneous weather observations along the South African coast for storm warning purposes. His meteorological activities were reported at meetings of the South African Philosophical Society, of which he was a foundation member. He served on the society's council from its formation in 1877, and was its president from 1881 to 1883. At a meeting in March 1881 he compared the temperature and rainfall on top of Table Mountain with observations made at the Royal Observatory, showing that in the previous two months the rainfall on Table Mountain was nearly four times as much as at the observatory. This work was published in the Quarterly Journal of the Royal Meteorological Society (1881). Several other papers were published in the Transactions of the South African Philosophical Society: "Rainfall in South Africa" (1877-1879, Vol. 1, pp. 73-82) provided a general review of the distribution of rainfall in the south-western Cape; "Summer and winter temperatures in South Africa" (1879-1881, Vol. 2, pp. 104-109) was a review of the published temperatures recorded at 22 meteorological stations; his presidential address, "The barometer and the winds" (1881-1884, Vol. 3, pp. xix-xxxvi), included a discussion of the factors causing winds, storms and anticyclones; there was also a "Catalogue of printed books and papers relating to South Africa. Part II. Climate and meteorology" (Vol. 3, pp. 151-195); and "Meteorological notes" (1884-1888, Vol. 4, pp. 10-14), a discussion of monthly mean temperatures and atmospheric pressure at coastal and inland stations. His derivation of heights above sea level from barometric observations led to an early paper in the Cape Monthly Magazine (second series, 1876, Vol. 13, pp. 124-126; addendum p. 359), and a report to parliament, Altitudes above sea level of places in South Africa south of 20 degrees South latitude (1886). Papers on "The rainfall of the Cape of Good Hope" appeared in the Meteorological Magazine in 1885 and 1886, and a further one on the rainfall on and around Table Mountain in the Quarterly Journal of the Royal Meteorological Society (1888).
Gamble's scientific interests extended way beyond meteorology and hydrology however. For example, in 1876 he presented ants from Calvinia and some beetles from Victoria West to the South African Museum; in February 1880 he exhibited and explained the working of a calculating machine (constructed by someone else in 1873) at a meeting of the South African Philosophical Society; and at a meeting in April 1882 he exhibited and explained the working of "a model of a sun engine" (a solar cooker). His paper on "Some problems awaiting scientific research in South Africa" (Transactions, Vol. 2, pp. 91-99) also dealt with problems outside his main interests. He was an examiner in science for the University of the Cape of Good Hope during 1876-1886, setting papers at the BA, BA (Hons) and MA level in plane trigonometry, calculus, algebra, coordinate geometry, statics, dynamics, geometrical optics, and hydrostatics. He served on the council of the university during 1885-1887.
In 1878 Gamble married Constance Brounger, daughter of the railway engineer William George Brounger*. They had three daughters. Upon leaving the Cape Colony he returned to England and was soon appointed to the newly created post of chief hydraulic engineer of Ireland. His active life was cut short by his untimely death as a result of typhoid fever at the age of 47.