Gage served in World War I as a Captain of the Coldstream Guards, seeing action in both France and Belgium.
He was appointed Honorary Colonel of the 58th (Home Counties) Field Brigade, Royal Artillery, on 17 June 1936.
Gage later (between 1958 and 1965) donated letters and documents from ancestral in-law Vice-Admiral Sir Peter Warren to the Sussex Archaeological Society in Lewes.
The couple decided to showcase the art in their home, opening Firle Place to public tours in 1954.
A stained glass window by the artist John Piper was installed in St Peter's Church, Firle, and dedicated to his memory.