So, who were William Gammell’s parents? We don’t know for sure, but below are a number of possibilities and what follows is a stream of consciousness based on my current research of the records.
Starting with what we do know.
The first definitive fact we have about William Gammell (Gemmil) is that he married Margaret Scott on 31 August 1727 in Greenock.
Family tradition has it that William was born in 1695 and an entry in an old family tree, probably compiled in the early 19th century, indicates that he had a sister, Agnes, who married Wm Thomson in Netherton and that they had 2 children, Andrew Thomson, farmer in Netherton, and Alexander Thomson shipmaster in Port Glasgow.
If he was really born in 1695, he was married at the age of 32, not by any means impossible, but possibly older than usual?
There are 14 William Gammell (but spelt in various ways) births recorded (on Scotland’s people) between 1690 and 1710 (dates which would have made him between 17 and 37 when he was married, which seems a reasonable assumption).
It is striking that all these locations (excepting Kilsyth) are either in Glasgow or south-west of Glasgow. Of these Williams, clearly number 6 looks the most likely, but looking at death records, it appears possible that he died on 10/01/1716? It is further possible that number 2 died in infancy, given that his parents named another child William two years later? It is also possible that 4 and 5 died on 17/04/1725 and 05/03/1703 respectively? If that were the case (which it might not be), they obviously could not be the William Gemmil who married Margaret Scott in 1727.
Looking now at Agnes Gammell births in the period 1680 to 1720.
There seems to be no commonality between the William and Agnes children, meaning that from the information available, it appears to be impossible to find a family that had a William/Agnes brother and sister.
Also from the information available, no William Thomson married a Gammell (or similar) between 1710 and 1740, though an Elizabeth Gemmil married Robert Thomson on 10/12/1713 in Kilmarnock, an Agnes Gemmel married Robert Thomson on 17/06/1714 in West Kilbride, an Agnes Gemmil married Hugh Thomson on 02/07/1719 in Kilmarnock, and a Janet Gemmil married Thomas Thomson on 20/11/1734 in West Kilbride.
Of the candidate families available, probably the most likely is Androw Gemell and Jean Hendrie of Fenwick, who had a son, William, on 27 October 1695. To believe it is the same William, we would have to ignore the death of a William Gammell in Fenwick on 10 January 1716, assuming that he was another William (which he might or might not have been).
If we do that, we know that Androw Gemell and Jean Hendrie had two other children Andrew, born on 13 February 1698 and Janet, born on 23 February 1701 and it is interesting that a Janet Gammell married a Thomas Thomson on 20/11/1734.
But to connect these without further information is pure guesswork, and certainly cannot be done with any confidence!