The last we hear of William Gibson is in the 1881 Census Record where he is listed, with his brother Finlay, residing at their sister Margaret's home in Stevenston. Finlay married and went to live elsewhere in Stevenston but it is less clear what William's next move was. He was not listed in Stevenston in 1891.
There is a possibility that he went back to his old haunts in the East End of London and this may be William and family in 1891:
Name: William Gibson
Event Type: Census
Event Date: 1891
Parish: St George In The East
Ecclesiastical Parish: ST JOHN
Registration District: St George In The East
Residence Note: Great Hermitage Street
Marital Status: Married
Occupation: Wharf Labourer
Relationship to Head of Household: Head
Birth Year (Estimated): 1841
Birthplace: London, England
Page Number: 29
Registration Number: RG12
Piece/Folio: 288/ 116
Household Role Sex Age Birthplace
William Gibson Head Male 50 London, England
Sarah Gibson Wife Female 40 Kent, England
Nellie Gibson Daughter Female 8 London, England
Rosina Gibson Daughter Female 5 London, England
John Foreman Lodger Male 46 Essex, England
The birth year and age fit - he was 40 in the 1881 Census, here he is 50. The strongest clue is mention of St George in the East which was William's district before he joined the Army. His occupation is given as 'Wharf Labourer' and he was a 'labourer' when he enlisted so that is not too much of a stretch.
He had clearly married, and his first child was 8 in 1891. So he could have started his family about 1883. There were two daughters and a lodger, the latter no doubt a helpful addition to the family income.
However, this is speculation and apart from the clues as to birth year, parish and occupation more information is required to be certain of the identity of this William Gibson.
Map showing St George in the East - the proximity of the River Thames
makes it a good location for a 'wharf labourer'.