John Gaff Gillespie was born on 17 September 1870 at 76 Abbotsford Place, Tradeston, Glasgow, the eldest of at least nine children of Alexander Gillespie, a Gorbals baker who originated from Duntocher, and his wife Margaret Gaff from Polmont. He was articled to James Milne Monro from 1886 to 1891, concurrently attending classes at Glasgow School of Art, and he won the Glasgow Institute of Architects prize in 1889 jointly with Charles Rennie Mackintosh. This brought him to the notice of William Forrest Salmon, by then in charge of his late father's firm James Salmon & Son, who engaged him in 1891. Like Mackintosh at Honeyman & Keppie, Gillespie was given design responsibility very early, notably at the free Flemish Renaissance Scottish Temperance League building in 1893 and the West of Scotland Convalescent Seaside Homes at Dunoon in 1895, by which time Gillespie was in charge of most of the design work. He was made partner in the same year, the everyday work of the practice having grown as a result of Forrest having secured some of the business of the British Linen Bank, whose architects were usually J M Dick Peddie & Washington Browne.
Forrest's son James Salmon (Junior) joined the family firm in March 1895, having previously commenced his apprenticeship there before moving to William Leiper's office and subsequently touring the continent. For the next few years and even beyond the individual design responsibilities of Gillespie and James Junior are not always easy to separate. James became a partner in 1898, but neither his nor Gillespie's name was acknowledged in the practice title until November 1903 when the firm became Salmon Son & Gillespie. Unlike the diminutive James Junior, Gillespie was very tall, slim and cleanshaven with a calm equable temperament.
Probably due to the influence of Forrest Salmon, who was a Governor of Glasgow School of Art, President of the Glasgow Institute of Architects 1892-94, and a member of the RIBA Council, both Gillespie and James Junior were admitted directly to Fellowship of the RIBA on 3 December 1906, Gillespie's proposers being Monro, Thomas Lennox Watson, Leiper and Forrest Salmon. By this time both Gillespie and James Junior had travelled extensively, Gillespie's nomination paper recording travel in Italy (one month in 1902) and Spain and Morocco (one month in 1905).
By the early 1900s Gillespie and Salmon's styles had begun to diverge, Gillespie's work tending to be a simplified free classic and Salmon's still a sculpturesque art nouveau as seen in the alternative elevational treatments in the competition for the new Glasgow and West of Scotland Technical College competition of 1901, both partners adopting a highly simplified arts and crafts style for domestic, cottage hospital and golf clubhouse work. But from 1904 when they received the commission for Lion Chambers both Gillespie and James Junior had become interested in the possibilities of reinforced concrete, working closely with the structural engineer Louis Gustave Mouchel, the British-based representative of Francois Hennebique. Within the firm Forrest seems to have been responsible for the 'scheming out' of commissions, the detailed design work being delegated to Gillespie or to his son James. Forrest was latterly known as the commercial traveller adept at moving in Parish Council School Board and clubland circles to obtain directly commissioned work for the practice which spent much of its time on designs for national and local competitions, none of which it succeeded in actually winning until 1908 when William Leiper selected their design for Stirling Municipal Buildings which was mainly Gillespie's work. Construction was, however, some years away and in the summer of 1911 Forrest began to suffer from cancer; he died on 7 October.
While the Finance Act of 1909 had probably affected the prosperity of the practice as it had so many others, Forrest Salmon's will proved the catalyst for the dissolution of the partnership in June 1913. The will made no provision for James to inherit his share of the practice; instead, it remained part of his trust estate and entitled his stepmother, Forrest's second wife Agnes Cooper Barry, to a share of such profits as the firm had at that time. Gillespie now became senior partner and as James Junior had spent all his income on foreign travel and motoring (as a letter to his brother Hugh of 18 August 1910 records) he could not afford to buy out either Gillespie or his stepmother. Gillespie bought out Agnes's trust estate interest, retaining the office in Mercantile Chambers, the archive and the Stirling commission. James moved out to a rented flat at 48 Jane Street, Blythswood Square which was both home and office, apparently without even a secretary. He retained the commission received in 1909 for the Admiralty Village at Cove Farm, Greenock of which only a few houses had been built in 1910, and was allowed to revive the name of the firm as it had existed prior to 1903, James Salmon & Son, later abbreviated simply to James Salmon FRIBA.
Gillespie died on 7 May 1926, leaving estate of £1,950 4s 11d to his wife Agnes Harriet Spencer, whom he had married late in life on 14 September 1923 at 4 King's Park Avenue (then his or her house). His executor was William Alexander Kidd, his partner since 1918. Born in 1879 he had joined the practice in 1898 as an apprentice from Greenock. He became chief draughtsman sometime before 1911, when he was admitted LRIBA, his proposers being Salmon, Gillespie and Harry Edward Clifford. Kidd continued the practice under the same name and a year later took into partnership Jack Antonio Coia, who had joined the firm in 1915 before working in several London offices, and had only recently returned to Glasgow. Kidd died later that same year, and Coia continued the practice alone under the same name of Gillespie Kidd & Coia. The entire archive of the Salmon practice was sent for pulping when Coia was interned in 1940.