Since it's raining, I just kept on looking
Here is Alex Sr. and family in 1881. Ancestry has the households all mixed up in their index, but I finally got it straight, I think...
1881
Main St Village East Side, Latheron, Caithness
Alexander Gilchrist, 51, Head, born Lathern, Caithness, Carrier
Ann Gilchrist, 48, Wife, born Lathern Caithness, Carrier's Wife
Charles Gilchrist, 27, Son, born Lathern, Caithness, Carrier
And Margaret & Williamina. Again, the households are a jumble in the index, you have to go through and hunt for the household and page
Here, at least, the Gunn motif is introduced, as I assume that's what William's surname really is 
1881
Farm Servts House, Olrig, Caithness
William Gann, 32, Head, born Halkirk, Caithness Shire, Farm Servant
Williamina Gann, 28, Wife, born Latheron, Caithness Shire, Farm Servs Wife
Ann S Gann, 6, Daughter, born Olrig, Caithness Shire, Scholar
Margaret Gann, 4, Daughter, born Thurso, Caithness Shire, Farm Servs Daur
Rose Gann, 3, Daughter, born Thurso, Caithness Shire, Farm Servs Daur
Margaret McBeath, 61, Mother-in-law, born Latheron, Caithness Shire, Farm Servs Mother In Law
To tidy things up, this looks like Alexander Sr in 1851 and 1841. Interesting that he didn't name either of his sons for his father. Do you suppose Ann MacGregor, the "visitor" is the one who later married Alexander?
1851
Smerby Lybster, Latheron, Caithness
George Gilchrist, 73, Head, born Lothbeg, Sutherland, Farmer of 7 Acres
Betty Gilchrist, 26, Daughter, born Latheron, Caithness, Farmer's Daughter
Alexander Gilchrist, 22, Son, born Latheron, Caithness, Farmer's Son
John Gilchrist, 16, Son, born Latheron, Caithness, Farmer's Son
Ann Gilchrist, 7, Daughter, born Lybster, Caitness, Scholar
Ann MacGregor, 23, Visitor, born Latheron; Wheel, Caithness, Formerly House Servant
Morison Snody, 3, Grandson, born Lybster, Caithness
Elizabeth Sutherland, 3 weeks, Grandaughter, born Lybster, Caithness
1841
Smerley, Latheron, Caithness
George Gilchrist, 60, born Caithness, Farmer Cancer
Ann Gilchrist, 40, born Caithness
Elizabeth Gilchrist, 15, born Caithness
Jane Gilchrist, 13, born Caithness
Alex Gilchrist, 11, born Caithness
William Gilchrist, 8, born Caithness
I wondered where John from the 1851 had got to and found that Ancestry had put him next door with the Clark family, since he was on the next page
thanks to FreeCen for this (although the surname in FreeCen is Gilchriss)John Gilchrist, 6, born Caithness
Also next door with the Clarks is a young man named Alexander Sutherland, age 15. Could he be the future father of Elizabeth Sutherland who was 3 weeks old in 1851 above?
There are a number of Margaret McBeaths in Latheron Parish in 1841, as you can see on FreeCen. Since the age was rounded down, it is hard to be certain which one is the correct Margaret. Three of them look possible. A peek at her death certificate might solve this.
So, when did Margaret die? Jumping forward to 1891, we find Williamina and now 8 children and father in law
But no sign of mother Margaret. Were Alexander's parents both deceased on his death cert?1891
Newlands, Olrig, Caithness
William S Gunn, 38, Head, born Halkirk, Caithness-Shire, Farmer
Williamina Gunn, 38, Wife, born Latheron, Caithness-Shire
Rose Gunn, 13, Daur, born Thurso, Caithness-Shire, Scholar
Susan Gunn, 9, Daur, born Olrig, Caithness-Shire, Scholar
Alexina Gunn, 8, Daur, born Olrig, Caithness-Shire, Scholar
Walter Gunn, 6, Son, born Olrig, Caithness-Shire, Scholar
William Gunn, 5, Son, born Olrig, Caithness-Shire, Scholar
Isabella Gunn, 3, Daur, born Olrig, Caithness-Shire
James Gunn, 1, Son, born Olrig, Caithness-Shire
Charlotte Gunn, 1Month, Daur, born Olrig, Caithness-Shire
Donald Gunn, 80, Father, born Halkirk, Caithness-Shire, Annuitant
There are two marg* mcbeath born 1818 +/- 3 years
One in Olrig, where she was living in 1881, the other in Thurso
The one in Olrig died in 1889 and was born in 1818. Looks about right, so she would have still been living at her son Alexander's death
Papa Alexander is also still living:
1891
Village, Latheron, Caithness
Alexander Gilchrist, Head, born Latheron, Caithnessshire, Carters
Anne Gilchrist, 58, Wife, born Latheron, Caithnessshire, Carters Wife
And in 1901, we get confirmation that he was, indeed, the father of Williamina, as her daughter is in the household as Grandchild
1901
Lybster Vilage Main St
Alexr Gilchrist, 71, Hea, born Latheron, Caithness, General Carrier
Annie Gunn, 26, Grandchild, born Olrig, Caithness, Domestic Servt
Kenneth McLeod, 20, born Latheron, Caithness, Carriers Assistant
William and Williamina finally named a son after his father, but both William and Donald have died. She is left with 7 children still at home (out of a total of 12 if I have counted correctly
) and a grandson...1901
Newlands East Murkle, Olrig, Caithness
Williamina Gunn, 50, Head, born Latheron, Caithness, Farmer
Susan Gunn, 19, Daur, born Olrig, Caithness, Farm Serv
William Gunn, 15, Son, born Olrig, Caithness, Ploughman
Isabella Gunn, 13, Daur, born Olrig, Caithness, Scholar
James Gunn, 11, Son, born Olrig, Caithness, Scholar
Charlotte Gunn, 10, Daur, born Olrig, Caithness, Scholar
Donald Gunn, 8, Son, born Olrig, Caithness, Scholar
John Gunn, 6, Son, born Olrig, Caithness, Scholar
William Smith, 4, Grandson, born Olrig, Caithness
Have fun with all this!
Sarah