In the 1960s, Gilbert compiled some historical atlases. His other major works include a single-volume history on the Holocaust, as well as the single-volume histories "First World War" and "Second World War". He also wrote a three-volume series called "A History of the Twentieth Century". Gilbert described himself as an "archival historian" who made extensive use of primary sources in his work. Interviewed by the BBC on the subject of Holocaust research in 2005, Gilbert said he believed that the "tireless gathering of facts will ultimately consign Holocaust deniers to history."