George F. Gilfillan was the son of William Frederick Anderson Gilfillan and his wife Anna Margaret Thornhill. He was boring for coal for the government of the Cape Colony near the mouth of the Cowie River in Lower Albany in January 1869, when he was sent to the region along the Orange River where the Hope Town diamond had been found in 1867. His brief was to investigate why Mr James R. Gregory* of London, who had recently visited the Cape, had concluded that the geology of Griqualand West made it very unlikely that diamonds would be found there. Gilfillan reported his initial impressions in a letter to the Grahamstown Journal, published on 23 January 1869. He found that Gregory had not been near the diamond fields at all, but had crossed the river downstream and then proceeded to Griquatown. During his investigation Gilfillan handled 18 diamonds, supporting Dr W.G. Atherstone's* opinion that many more diamonds would likely be found in the region. He compiled a plan of the diamond fields which, though incorrect, was used by Sir Henry Barkly* in the dispute with the Orange Free State over ownership of Griqualand West. He described the results of his investigation in the form of a short paper, "On the diamond district of the Cape of Good Hope", in the Quarterly Journal of the Geological Society of London (1871, Vol. 27, pp. 72-73). As co-author with John Shaw* he furthermore participated in writing an article, "On the diamond fields", for the American Journal of Science (1871).
Meanwhile Gilfillan had returned to boring for coal in Lower Albany. In the Report of the Chief Inspector of Public Works for 1869 he reported on the carbonaceous shales encountered near present Port Alfred, providing a plan and section. The next year he described his continued search for coal in the area in the Report for 1870. He probably also visited Natal at some time, for there is a manuscript titled "Geology of Natal", written by him and dated 1870, in the T.R. Jones* collection of documents on the geology of South Africa. Later he wrote an article on "Springs: Their nature and mode of occurrence in this colony" for the Cape Monthly Magazine (2nd series, 1875, Vol. 10, pp. 307-311). On 1 August 1870 he married Margaret Buckley Ross. He died in 1878 (during the Ninth Frontier War) when he was struck by lightning while on patrol, leaving his wife and five small children.