G.G. Holmes, geologist and mining engineer, was an associate of the Royal School of Mines (London). By 1898 he was living on the Witwatersrand and in September that year married Georgina Isabel St John Maxwell in Krugersdorp. At that time he was employed as a mining engineer at the Porges Randfontein Gold Mining Company. In 1904 he was living in Johannesburg, where he became an active member of the Geological Society of South Africa, serving as vice-president in 1912, 1914 and 1915. He published five papers on the geology of parts of the Transvaal Colony, all five in the Transactions of the Geological Society of South Africa during 1904-1906. In the first, "Some notes on the geology of the northern Transvaal" (1904, Vol. 7, pp. 51-56) he described the distribution of the Waterberg Group, which was being studied by several geologists at the time. He found that in the north of the territory it rested directly on old granite and schists, and confirmed that there is a distinct break between it and the Pretoria Group. His observation of red felsite boulders and pebbles in the basal conglomerate of the Waterberg Supergroup furthermore showed that at least one period of eruption of the Bushveld Complex predated its formation. In "The geology of a part of Bechuanaland west of Vryburg" (1904, Vol. 7, p. 130) he found that the Black Reef Quartzite Formation and the dolomites of the Transvaal Sequence cover a large area and extend southwards and westwards as far as Taung and Kuruman. Two further papers dealt with "The geology of a part of the Rustenburg District" (1905, Vol. 8, pp. 1-6) and "The Pretoria Series in the Marico District" (1906, Vol. 8, p. 167-174). In his final paper, "The geology of the south-western Transvaal" (1906, Vol. 9, p. 90-96) he reported finding the most northerly exposure yet of the Ventersdorp Supergroup, near the Transvaal-Botswana border, where diabase appears from under the syncline formed by the Black Reef Quartzite Formation and dolomite. He also found the amygdaloidal diabase and volcanic breccias of the Ventersdorp Supergroup lying between the Black Reef Formation and the older granite in the vicinity of Vryburg.
Holmes was still a member of the Geological Society of South Africa in 1919. He became a member of the South African Association for the Advancement of Science in 1916.