quick scattergun approach to start your quest
Gilston in Co Down almost unheard of, but Gelston has about 50 entries in
http://freepages.genealogy.rootsweb.ancestry.com/~rosdavies/SURNAMES/G/Ge.htmback into Belfast using street directories
http://www.lennonwylie.co.uk/1819_Alphanames.htmfrom
Gelston, John, cabinet maker, 6 Mitchell's Entry
Gelston, Elizabeth, leather cap maker, Chichester Street
Gelston, Samuel, cabinet maker, 41 Waring Street
Public Record Office for Northern Ireland ecatalogue
Letter from Sam Boyce to [his sister,] Mary
Dates : 2 July 1830
Description : Letter from Sam Boyce to [his sister,] Mary Gelston, 54 Tomb Street, Belfast describing his progress as a buyer and seller of linen and referring to Mary's inclination to change her religion.Copy of will and probate of William Gelston, merchant of St George's Street, Belfast. 1864
Public Record Office for Northern Ireland
PRONI Reference : D46/1Index to Report of Belfast Special Constables of Nightly Watch transactions, Vol. 1: pages 1-580 (27 November 1812), Vol. 2: pages 581-653 (16 May-21 October 1816).
Gelston, Henry - Page 487, 573, 637,
Public Record Office for Northern Ireland
PRONI Reference : D556/17 Title : Lease for five lives or 41 years £23.15s.
Dates : 29 December 1787
Description : Lease for five lives or 41 years £23.15s. of Tullycarnet, Parish of Knockbreda, Co. Down; a map attached shows portion of the land: North of Belfast Comber Road, adjoining on west is land of Hugh Montgomery, Esq.; on east land of Robert Gelston and on the north 'Ballycloghan' from Charles Ranken Esq., Captain in the Service of the East India Company to William Gelston of the Knock, Parish of Knock Breda, Co. Down, farmer - Witnesses: Robert O. Black and Henry Joy.
there maybe/are further entries in these starting websites
hoping others join in