A number of references to Gow portraits can be gleaned from the pages of the Scotsman.
This book might be worth a look. An advertisement by ALEX ROBERTSON AND CO., MUSICSELLERS TO THE QUEEN, 30 Princes Street announced: ‘Just Published A CHOICE SELLECTION of REELS and STRATHSPEYS being the original arrangements from the Works of Gow and MARSHALL with the most popular Country Dances, and a portrait of NIEL GOW…’ (23 June 1855, p. 1)
On Saturday May 5 1877, Chapman’s Auction Rooms, 11 Hanover Street, Edinburgh held a sale of ‘Pictures of the Highest Class’ including a ‘Portrait of Niel Gow by Sir Henry Raeburn, R.A.’ (4 May 1877, p. 8).
In 1884 there was an advertisement: ‘Lyon and Turnbull. To-Morrow (Saturday) VALUABLE COLLECTION OF OIL PAINTINGS, WATER-COLOURS, DRAWINGS, ENGRAVINGS, etc. The sale will include and Original portrait of Niel Gow, by Alex Carse, painted from life at Inver, near Dunkeld, “The Stirrup Cup” with Portraits of Robert Burns, William Nicol and Niel Gow, by Stewart Watson….’ (Scotsman 11 April 1884 p. 8)
This Carse portrait is a new one to me. The Stewart Watson is almost certainly extant as we know this sale took place at Christie’s sale of Modern and Victorian Scottish Paintings and Drawing held at Glasgow on September 17 1987:
The Stirrup Cup, at Inver near Dunkeld, 1789
Lot 104 $ 7,901 USD / £ 4,800 GBP
Materials: oil on canvas
Measurements: 31.00 in. (78.74 cm.) (height) by 43.00 in. (109.22 cm.) (width)
More recently there was an announcement ‘The London Salerooms. Christie’s art sales restart tomorrow… Later there will be a dispersal of old porcelain and Louis XV and Louis XVI furniture owned by Colonel W. S. Balfour, D.S.O., of Balbirnie, Markinch, Fife; and pictures to be sold on October 7 include a portrait of Niel Gow, playing a violin, attributed to Raeburn…’ (Scotsman 27 September 1949 p. 4)