John Gannon was born in Ireland c. 1789.
He was appointed Assistant Surgeon on 7 January 1811.
He was employed as Assistant Surgeon on the Medina in 1821-22 
John Gannon was employed as Surgeon-Superintendent on two convict ship voyages to Australia:
1). Asia V
to New South Wales in 1837.
John Gannon joined the Asia at Deptford on 28 June 1837. He kept a Medical Journal from that day until 11 December 1837 -
The prevailing diseases that he had to deal with were catarrh, diarrhoea, inflammatory fevers, some ring worm, and there was one case of confirmed phthisis pulmonalis. They crossed the equator on 17 September and found the South East trades on 18 September. There were catarrhs, sore throats, diarrhoea and one case of chronic rheumatism at this time. The catarrhs and sore throats occurred in the prisoners who were on deck in the evenings, which were damp and sometimes chilly. In November the weather became boisterous with heavy showers of hail and gales and heavy seas but was clear and dry from the 8th. On 24 November they sighted the West Coast of New South Wales. Prevailing diseases were catarrh, rheumatism and several severe cases of scurvy 'caused by the cold, damp and heavy state of the weather, and the 'wet state of the ship' at the start of the month. There were two deaths on the voyage, one from consumption and the other from apoplexy. 
2). Barossa to Van Diemen's Land. The Barossa departed the Downs on 17 May 1844 and arrived in VDL on 5 September 1844. John Gannon kept a Medical Journal from 5th April 1844 to 28 September 1844. There were two deaths on the voyage out.
In the years 1841, 42 and 43 he was employed as surgeon on H.M.S. Monarch . The Monarch, Samuel Chambers in command served in the Mediterranean.  Read about illness on board H.M.S. Monarch in Souvenirs and New Ideas edited by Diane Fortenberry
In 1851 John Gannon is recorded residing at Chelsea. He is a widower, age 62 and lodger at the house of Jane Cox. He gave his occupation as retired surgeon, Royal Navy. Place of birth - Ireland.
John Gannon died at Clifton, England on 27 August 1861 -
The Will of John Gannon formerly of Smith street Chelsea, Co. Middlesex but late of 20 Sion-hill Clifton, City and County of Bristol a Surgeon in the Royal Navy deceased who died 27 August 1861 at Sion-hill aforesaid was proved at the Principal Registry by the oath of Matilda Lucretia Palmer of Chelsea, spinster, one of the Executors.
 The Medico-chirurgical Review, and Journal of Medical Science
 United Services Magazine
 The New Navy List
- The Names of all the Commissioned Officers in Her Majest's Navy By Charles Haultan
 Ancestry.com. England and Wales, National Probate Calendar (Index of Wills and Administrations), 1858-1966, 1973-1995
 Ancestry.com. UK, Royal Navy Medical Journals, 1817-1857. Medical Journal of John Gannon on the voyage of the Asia in 1837. The National Archives. Kew, Richmond, Surrey.