Nicolas Joseph Gillet was an engineer resident in Cape Town. In 1887 or 1888 he undertook a journey from Johannesburg to Malmani (near Ottoshoop in the western Transvaal) where a gold-field had been proclaimed in February 1887. He returned via Johannesburg to Cape Town. This trip formed the basis for a lecture on "The Transvaal gold-fields: their origin and formation", which he delivered in Cape Town on 15 August 1888 and which was published as a pamphlet. Two years later he published another pamphlet, The Prince Albert gold-fields (Cape Town, 1891), with a map of the district. Meanwhile he had also produced an 80-page monograph, Water finding and artesian well boring. The means of finding water everywhere (Cape Town, 1890).
Gillet was still in the Cape Colony at the start of the Anglo-Boer War late in 1899, when he compiled a propagandistic publication, With Methuen to Kimberly: the advance reviewed by an eye-witness (Cape Town, 1900). He applied to become a naturalized citizen in 1902. For several years from 1897 he was involved in a case against the Commissioner of Public Works about the use of water on the farm Klein Kruidfontein in the Division of Beaufort West. The General directory of South Africa for 1903 lists N.J. Gillett (with two t's), a "contractor", as living in Cape Town. This is presumably the same person.