I have to say I haven't delved very far back into Janet's ancestry at all really, other than her parents who were James Gall and Janet Crabe/Craib. I presume you have all her other siblings (there were 6 children altogether - 3 boys and 3 girls): James 1726, Alexander 1729, William 1731, and Isabel 1741. Thanks for inadvertently providing me with the name of Isabell Bruce's husband, Patrick Chizzet (William's sister).
Don't know if you have this info. It's from the baptism records (provided to me by another Bruce chaser):
Jannet's surname is spelled "Gaa" in baptisms of five children:
1 Isobel BRUCE born Thorntown, bpt Grange 12 January 1768.
Witnesses "William Keith in Farmtown, William Gall, Isobel Syme & Isabel Gaa"
2 Margaret BRUCE b Thorntown, bpt Grange 5 May 1770. (named after Alexander’s mother)
Witnesses "Alexr Chizzat & Alexr Shand, Margt Keith, Margt Gaa"
3 William BRUCE b Thorntown, bpt Grange 21 May 1772.
Witnesses "William Brown Schoolmaster at Grange, Anna Innes at Braco, Miss Katherine [rest illegible, discoloured and torn at bottom of page]"
4 Alexander BRUCE's baptism has no date. It is squeezed in very small handwriting between entries for January 17th and January 20th 1774:
"Alexander Bruce in Thorntown had a Child by his wife Jannet Gaa baptised and called Alexander. Witnesses Alexander Thomson, Jean Shand, Thorntown, Alexander Gordon, Craiglethy"
5 James BRUCE b Kirktown of Grange, bpt Grange 13 January 1780.
Witnesses "James Hui in Nethermiln and James Shand in Loch of Skeen and James Martin in Kirktown" (Loch of Skeen may have been in Forgue.)
I suspect that a lot of records have been lost in the past, due to fire or whatever. Research can be a bit of a lottery at times.