On 29 May 1911, Gilbert was about to give a swimming lesson to two young women, Winifred Isabel Emery (1890–1972), and 17-year-old Ruby Preece in the lake of his home, Grim's Dyke, when Preece got into difficulties and called for help. Gilbert dived in to save her but suffered a heart attack in the middle of the lake and died at the age of 74. He was cremated at Golders Green and his ashes buried at the churchyard of St. John's Church, Stanmore. The inscription on Gilbert's memorial on the south wall of the Thames Embankment in London reads: "His Foe was Folly, and his Weapon Wit". There is also a memorial plaque at All Saints' Church, Harrow Weald.