Hi Andy, for brief moment I wondered if your mother could have been a Sheila Leslie who is in my tree, but the dates don't fit.
One missing piece of your information above is the parentage of Mabel Mary Jane Robb. If you don't have her birth certificate you can download a copy of it at
www.scotlandspeople.gov.ukI see that she was born in Aberdeen St Machar in 1905. I also see that the 1911 census index lists in St Machar John Robb, 40; Christian Robb, 44; John Alex Robb, 6; and Mabel M J Robb, 5.
There is nothing remarkable about Christian Forbes Gall's mother's maiden surname. Robb is a fairly common surname and there is no reason why John Gall's daughter should not have married a man with the same surname as her mother. They could even have been related before they married.
John Gall and Mary Robb had at least 8 children
Mary Johnston, born 23 December 1864, Aberdeen St Nicholas
Christian Forbes, born 26 November 1866, Aberdeen St Nicholas
John, born 26 November 1866, Aberdeen St Nicholas
Isabella, born 30 May 1869, Aberdeen Old Machar
Margaret, born 1871, Aberdeen Old Machar
Sarah Ann, born 1873, Aberdeen Old Machar
Robert Robb, born 1877, Aberdeen Old Machar
James, born 1880, Aberdeen Old Machar
and there could have been more children born after the 1881 census
The family are in the 1881 census at 2 Westfield Place, Aberdeen
John GALL M 43 M Kennethmont, Aberdeen, Scotland
Rel: Head
Occ: Master Mason Employing 6 Men
Mary GALL 46 F Old Deer, Aberdeen, Scotland
Rel: Wife
John GALL 14 M Aberdeen, Scotland
Rel: Son
Occ: Errand Boy (Seedsman)
Christian F GALL 14 F Aberdeen, Scotland
Rel: Daur
Occ: Scholar
Margaret GALL 9 F Aberdeen, Scotland
Rel: Daur
Occ: Scholar
Sarah A GALL 7 F Aberdeen, Scotland
Rel: Daur
Occ: Scholar
Robert R GALL 4 M Aberdeen, Scotland
Rel: Son
James GALL 5 m M Aberdeen, Scotland
Rel: Son
So both John Gall and his wife Mary Robb were from rural parts of Aberdeenshire. The 'community contributed' section of the IGI at
www.familysearch.org says they were married at Kinnethmont on 18 June 1864. However this not not a trustworthy source so any information from it
must be verified from documentary evidence. (The 'community indexed' section is much more reliable)
Presumably you have the names of the parents of John Robb b 1871 from his marriage certificate. How, exactly, does it list his mother's name? Is she simply 'Jane Robb' or is she 'Jane Robb M S Robb' or is she 'Jane Robb M S ....' with another surname where I have put the dots?I see that John Robb and Jane Robb, maiden surname Robb, were married at Kildrummy on 24 July 1864 and had at least ten children
James, born 17 June 1864
John, born 24 February 1866
Alexander, born 19 October 1867
Jane Helen, born 24 October 1868
Ann, born 20 May 1872
Francis Frederick, born 15 October 1873
David, born 1875
William, born 1876
Harry, born 1878
Helen, born 1880
This family is in the 1881 census at Kildrummy Cottage, Kildrummy
John ROBB M 42 M Kildrummy, Aberdeen, Scotland
Rel: Head
Occ: Gamekeeper
Jane ROBB M 38 F Kildrummy, Aberdeen, Scotland
Rel: Wife
John ROBB U 15 M Kildrummy, Aberdeen, Scotland
Rel: Son
Occ: Gamekeepers Son
Jane ROBB 12 F Kildrummy, Aberdeen, Scotland
Rel: Daur
Occ: Scholar
Ann ROBB 9 F Kildrummy, Aberdeen, Scotland
Rel: Daur
Occ: Scholar
Francis F ROBB 8 M Kildrummy, Aberdeen, Scotland
Rel: Son
Occ: Scholar
David ROBB 6 M Kildrummy, Aberdeen, Scotland
Rel: Son
William ROBB 4 M Kildrummy, Aberdeen, Scotland
Rel: Son
Harry ROBB 2 M Kildrummy, Aberdeen, Scotland
Rel: Son
Helen ROBB 10 m F Kildrummy, Aberdeen, Scotland
Rel: Daur
and there could have been more children born after the 1881 census
You will have noted that this John Robb, son of John Robb and Jane Robb, was born in 1866, not 1871, so he does not quite fit as your John Robb, son of John Robb, sailor.
I am also puzzled why John Robb, a man who at the age of 40 had been a carpenter and had apparently lived all his life in the city of Aberdeen, should by 1928 have become a farmer in Auchnagatt, an area where he does not appear to have had any close connections.