Gilbert remained with the LPO until the outbreak of the Second World War in 1939, when he volunteered to join the Coldstream Guards. He remained nominally the orchestra's principal flautist until 1942, and managed to play in some concerts. He rejoined the London Philharmonic after the war (though Beecham was no longer its conductor), and became a teacher at the Guildhall School of Music and Trinity College of Music, London. His students included William Bennett, James Galway, Susan Milan, Stephen Preston and Trevor Wye.