Just looking at possible names of grandparents from children's names. Possibly Margaret Gunn's mother as Elizabeth/Betty, maybe maiden name Dunbar?
There is this 1851 entry:
Alexander Gunn 48 shepherd b. Kildonan, Sutherland
Betty Gunn 45 b.Thurso, Caithness
Margaret Gunn 10 b.Thurso, Caithness
Angus Gunn 7 b. Reay, Caithness
https://familysearch.org/ark:/61903/1:1:XBM2-ZWYBarbara Gunn 5 b. Reay, Caithness
Janet Gunn 2 b. Reay, Caithness
Peter Gunn 2 Months b.Thurso, Caithness
Elizabeth Dumbar 60, wife's aunt b. Halkirk Caithness
Address: Scrabster, Thurso
If this couple do turn out to be Margaret's parents, photo of their gravestone included here
www.oddquine.co.uk/gravestones/crosskirk/gunn.jpg from Crosskirk Cemetery, Caithness. Also includes brother Angus and family.
FreeCen now have 100% transcriptions of the Caithness censuses up to 1881 and building up now on the 1891 census. See
www.freecen.org.uk/cgi/search.plMonica