According to the tree on Ancestry, her Father was William Harris (1851 Great Linford, Bucks) and her Mother was Charlotte Brazier (1850 Wavendon, Bucks)
Have you applied for the Harris/Staggs marriage certificate?
These are the claimed siblings of Gertrude:
Margaret Ann Harris 1872 William James Harris 1875 Lily Harris 1876 Daisy Harris 1877
Walter Edward Harris 1879 Rose Harris 1880 Horace Marcus Harris 1881 Jessie Harris 1882
Minnie Harris 1884 Herbert Charles Harris 1885 Muriel Ivy Harris 1895