Gordon James Lethbridge-Stewart — or Gordy, as his mother called him — was the son of Kate Lethbridge-Stewart and a man named Jonathan. He was the grandson of Fiona and Alistair Gordon Lethbridge-Stewart. Doris Lethbridge-Stewart was his step-grandmother. (HOMEVID: Downtime; PROSE: Downtime; TV: Battlefield)
Gordon was born in late September 1990. He was brought up by Kate as a single parent, initially aboard a narrow houseboat moored on an English canal. His father, Jonathan, chose not to be a part of his and his mother's lives after Kate refused to marry him. (PROSE: Downtime)
Gordon was nearly five years old before he met his grandfather, Alistair, Kate having kept her pregnancy and Gordon's existence a secret from her father. Following recent events at New World University, Gordon acquired an imaginary friend called Danny and mentioned this on first meeting his grandfather. (HOMEVID: Downtime)
Gordon's first name was his grandfather's middle name. (HOMEVID: Downtime) He became the third Gordon Lethbridge-Stewart in his recent family history. (PROSE: The Forgotten Son)
Gordon gained a sibling later in life on his mother's side. (TV: Death in Heaven)
Behind the scenes
The in-universe article A Brief History of the Lethbridge-Stewarts, available as a download from the Lethbridge-Stewart website, says Gordon Lethbridge Stewart III "is making waves of his own". Because it is non-narrative, it is not a valid source on this wiki.