Hi Sue,
Sorry it has taken me so long to get back to you. I have done some digging and found the following...
In Castleton Churchyard, there is a stone with the following inscription:
"In memory of Donald Forsyth who dies at Bridge End Newcastleton 1902 aged 70 years. Also Janet Elliot wife of the above who dies at Newcastleton January 25th 1922 aged 84 years"
That would give you some more leeway on Donald's date of birth - circa 1832. I am also interested in how someone born in Avoch ended up in Castleton - it is a long way to travel.
I also checked the 1851 census and did not find any mention of a Gideon Elliot in either Castleton or Hawick. I did find one Janet Elliot aged 12 in the 1851 census for Castleton, but not sure if it is the correct one, as no parents are listed...
Dykecrofts
Helen Elliot, Daughter, Unmarried, 22, Farm Servant, Roxburgh Castleton
Mary Elliot, daughter, Unmarried, 20, Farm Servant, Roxburgh Castleton
janet Elliot, Daughter, Unmarried, 12, Farm Servant, Roxburgh Castleton
Robert Turnbull, Grandson, 12, Roxburgh Castleton
Agness Anderson, Granddaughter, 1, Roxburgh Castleton.
Unfortunately, there is no head of household given - so we are kind of drawing a blank there.
However, I did a quick search on the IGI at familysearch.org, and there are births of a Helen Elliot and Mary Elliot to Gideon Elliot and Agnes (or Nancy) Anderson at around the correct times - so it might be the correct family.
Sorry it is not better information - but it might help you on a little bit.
Cheers,
Clare