Thank you for posting this information about the definition of ND.
I am researching a man called William Thompson, who on the 1891 census, aged 52 and living in Northampton, is recorded as a Gentleman's Gardener too and I am trying to find out where he was working.
I believe I may have found William on the 1861 census, working as a house servant at 5, Cavendish Terrace, Clapham, Wandsworth, London & Surrey in the house of John Humphrey who is described as 'Housekeeper'.
In 1881, I believe I have found him again in Northampton at 42, Gt Russell Street, Northampton Priory of St Andrew Aka Town Part, Northampton, Northamptonshire, England (although he is ten years younger than I think he should be). He is recorded as a Gardener (nd) as described here.
William seem to have remained in Northampton after that and on the 1901 census, I have found a William Thompson at 42 Great Russell Street. His occupation aged 62 is given as Market Gardener and his wife Harriet is described as a Greengrocer/Shopkeeper. In 1911, aged 71, William was working as a Market Gardener too.
I too am interested in the idea that a man who appears to have been a servant in his 20's, later appeared to have the money to set up as Market Gardener and his wife as a shop keeper.