I don’t see a Johnston household in Gortnagullion in the 1901 census:
http://www.census.nationalarchives.ie/pages/1901/Fermanagh/Drumkeeran/Gurtnagullion/
Nor in 1911:
http://www.census.nationalarchives.ie/pages/1911/Fermanagh/Drumkeeran/Gortnagullion/
Nor is there a Johnston household listed there in the Valuation Revision records 1882 – 1898.
What evidence have you found linking the family to that townland?