He failed to win the Bruce electorate at the 1883 by-election, and won the seat at the general 1884 election by a majority of 217. He resigned in 1885 for failing health, which caused the 1885 Bruce by-election.
He died on 15 June 1886 from an aneurysm of the heart. He was the brother of Thomas Gillies and John Lillie Gillies, and the father of plastic surgeon Harold Gillies. Robert was buried in the Northern Cemetery.
He was one of 5 sons and 3 daughters of Isabella (née Lillie) and John Gillies (born in Rothesay on 22 April 1802). His father was a lawyer and a member of Otago Provincial Council. Robert spent a year at Glasgow University in 1851, but his father then decided to emigrate. They reached Otago on the barque, Slains Castle, on 6 November 1852 and bought 10 acres (4.0 ha) at Halfway Bush as a family home, and 100 acres (40 ha) at Riversdale, Milton.
In 1866 Robert married, Emily Street, the daughter of his business partner, Charles Henry Street (1824-1887). Emily was a niece of Edward Lear, the nonsense writer and landscape painter. Emily, who had moved to Dunedin with her parents and grandparents when aged 5, was a founder of the first kindergarten in Dunedin, and took a keen interest in charitable works. After Robert’s death in 1886, Emily moved her family from Dunedin to Auckland. By 1902 the children had grown up and she bought part of her mother’s Birtley estate. In 1905, the wedding of Emily Sophia Gillies (their eldest daughter) and Robert Williams Michell was held at her new Parnell house, Kohanga. In November 1911, Emily Gillies sold Kohanga, following the death of her mother, Mrs Street, earlier that year. She moved into Birtley, the home she had inherited from her parents, where she remained until her death on 7 September 1913, aged 65. Emily and Robert had eight children –
- Charles Stuart Gillies 1867-1905 helped found the Auckland Golf Club in 1894, was a lawyer and golf champion and died of meningitis
- George John Gillies 1869-1892, aged 22
- Robert Craig Gillies 1870-1925 was a sheep farmer at Hakataramea
- Arthur William Gillies 1871-1940 was elected mayor of Hāwera in 1912, started the development of Kawaha Point at Rotorua and became a stockbroker in Auckland
- Harry Thomas Gillies 1874-1950 was Crown Prosecutor at Hamilton for 36 years from 1910
- Emily Sophia Michell 1875-1929 was at Cambridge and a golf champion, who died in France
- Eleanor Lillie Bowen 1877-1939 was at Parnell, a golf champion and worked with charities for the blind in Cape Town
- Dr Harold D Gillies 1882–1960, in London, who was two days short of his 4th birthday when Robert died.