Interested to see our post. There were articles in the Clan Maitland newsletter some years back about this family. They do not add much to what you appear to know already, but I quote:
The family history is traced back to a Thomas Gray, farmer at Easterton Fyvie and of Westall, Oyne, Aberdeenshire, who died in 1805. By his wife Mary Cuthbert he had a daughter Janet Gray who married Adam Maitland, farmer in Balquhain, Aberdeenshire. This Adam Maitland is thought to be, although conclusive proof is not available, the Adam Maitland born in 1775, a son of Adam Maitland of Insch, manufacturer, who on 3rd July 1769 married Janet Mearns, the daughter of the Rev. Alexander Mearns, minister of Insch for 60 years.
I would be interested to know where you found Adam Maitland (1)'s date of birth.
There is, of course, a lot of information about some of the descendants, particularly Sir Herbert Lethington, surgeon, and Geoffrey Douglas, physiotherapist. But I would be grateful for dates of b, m & d of all the descendants. Perhaps you could e-mail me at firstname.lastname@example.org