G.A.P. Ross (also Park Ross, G.A., or Park-Ross, G.A.) came to South Africa in 1900 to serve as a trooper in the First Lovat's Scouts during the Anglo-Boer War (1899-1902). He was awarded the Queen's South African Medal with four bars. Upon his return to Scotland he qualified as Bachelor of Medicine (MB) and Bachelor in Surgery (ChB) at the University of Edinburgh in 1901 and the next year acquired the Diploma in Public Health (DPH) at the same institution. During 1902-1903 he travelled in the United States and Russia and in the latter year was appointed in the London Hospital for Tropical Diseases. This was followed by a period as medical officer to the Anglo-Portuguese Delimitation Commission demarcating the boundary between the Zambezia province of Mozambique and present Zambia and Zimbabwe. He was registered to practise medicine in Natal in 1906 and the next year was appointed district surgeon of Nqutu, Zululand, a post he held to 1910. Meanwhile he qualified as Doctor of Medicine (MD) at the University of Edinburgh in 1908.
During March-June 1910 Ross conducted research into the diseases of northern Zululand and was then appointed as assistant port health officer in Durban. The next year he became government pathologist and bacteriologist of Natal. During the re-organisation of the civil service following the formation of the Union of South Africa in 1910 he was appointed on 1 April 1912 as government bacteriologist at the Public Health Laboratory in Durban. He remained in the employ of the Department of Health until 1939 or later.
Ross first reported relapsing fever as a result of a tampan bite in South Africa. He observed the disease in Zululand in 1910 and described it in 'Human spirochaetosis' (Transvaal Medical Journal, 1912, Vol. 7, pp. 125-129). However, he is best remembered for his work on the control of malaria in South Africa, a topic on which he published a paper in the Medical Journal of South Africa in 1923. During the nineteen-thirties he led the campaign of pyrethrum spraying against indoor mosquitoes in Natal, in collaboration with Dr Botha de Meillon. Antilarval methods were found to be ineffective, but indoor spraying against adult mosquitoes worked very well. The results were reported in a paper entitled 'Insecticide as a major measure in control of malaria, being an account of the methods and organisations put in force in Natal and Zululand during the past six years'. The paper was read at the League of Nations Pan-African Health Conference held in Johannesburg in 1936 and published in the Quarterly Bulletin of the Health Organisation of the League of Nations (1936).
Ross married Frances Mary Smith in 1908. After her death he married Mary Lear.