After two years of postgraduate work, Gilbert was approached by Randolph Churchill to assist his work on a biography of his father, Sir Winston Churchill. That same year, 1962, Gilbert was made a Fellow of Merton College, Oxford, and became a part of the same circle of academics that included C.S. Lewis and J.R.R. Tolkien. He spent the next few years combining his own research projects in Oxford with being part of Randolph's research team in Suffolk, who were working on the first two volumes of the Churchill biography. When Randolph died in 1968, Gilbert was commissioned to take over the task, completing the remaining six main volumes of the biography.