As I think I have already said, I have not looked into the earlier generations of Gartshores because my possible connection is via Susanna Howie. Therefore I have no opinions on the antecedents of John Gartshore, or on the names of his siblings. I think he is the son of John G and Elizabeth Wood, baptised 11 May 1690 in the parish of Cumbernauld, but I am quite willing to be corrected if there is evidence to show that he was not this John Gartshore.
Susanna Howie (also in some records Anne or Anna Howie) married John Gartshore on 29 December 1712. I think it possible, if not likely, therefore, that she was born earlier than 1695. I am not certain who her parents were, but my 5th-great-grandfather Joseph Howie of Meikle Drumgray, who married Elizabeth Hay in Annathill on 29 July 1717, was a contemporary of Susanna/Anne Howie or Gartshore. Joseph's eldest son was Robert, so I speculate that Joseph and Susanna/Anne were related to or descended from Robert Howie, Portioner of Easter Glentore. He was the first of three Robert Howies, father, son and grandson. The son predeceased the father, because in a document dated 12 December 1710 the grandson is described as a minor, the only son and heir to the deceased Robert Howie younger of Glentore and grandson of Robert Howie there.
I am also related to later generations of Gartshores via Mary Mochrie, daughter of James Mochrie and Janet Waddell, who married Robert Gartshore, son of John G and Susanna/Anne Howie, in about 1752.
As for John G being born in Cumbernauld, there is no reason to think that he was. He was certainly baptised in Cumbernauld, and the original baptism record says, "John Gartshore son lawll to John Gartshore and Elizabeth having a testimony from East Monkland". Babies were sometimes baptised in a different parish, for example if the minister of their parents' home parish was absent or ill.
Blairlin has only been in the same local authority area as Cumbernauld since the reorganisation of local government in 1975, so this 'weird' fact is irrelevant, if not positively unhelpful, for purposes of historical or genealogical research.
I am very happy to exchange notes on the descendants of Robert Gartshore and Mary Mochrie, or to discuss the likely parentage of Susanna/Anne Howie or Gartshore, but for the present I have no reason to investigate the rest of the Gartshore family.