Gertrude's family were school teachers. Her mother, father and aunt were teachers and Gertrude and her sisters, Maude and Mabel became teachers. Gertrude was born on 8th September 1845 and was baptised later that month at St. Andrew, Holborn. Her father, Charles Emilius Johnstone, was 37, her mother, Eliza Johnstone, nee Hitchcock, was 33 and her sister Maude and Mabel were 3 and 2 years old. Her brothers, Charles and Edward were born in 1849 and 1853 respectively.
Gertrude moved with her parents to school at 32 London Street, Beaconsfield, where her parents were both teachers taking in male scholars include two of her nephews. In 1860, Following her father's death at Woodspeen, near Newbury, the family moved to Worthing and her mother set up a school at Marlborough House. All three sisters taught at the school. In 1877, her sister, Maude married, John Forbes Hardy, their landlord and artist. Before her mother's death in 1892, Gertrude moved to live with the Hardys' in their Manor Road, home called Beaconsfield. John Forbes Hardy died in 1906 and Gertrude, joined by her sister Mabel, moved to Maude's new home, Woodspeen, 7 Wordsworth Road, Worthing.
Gertrude died on 12th February, 1927 and was buried alongside Maude (d 1920) and Mabel (d 1925) in John Hardy's burial plot at Heene Cemetery following a funeral at Heene Church. She left her estate to her sister-in-law, Jessie Isabel Johnstone and her nephew, Rupert Johnstone, a clerk in holy orders.