Gilbert was present at the siege of Newhaven in Havre-de-Grâce (Le Havre), Normandy, where he was wounded in June 1563. By July 1566 he was serving in Ireland under the command of Sir Henry Sidney (then Lord Deputy of Ireland) against Shane O'Neill, but was sent to England later in the year with dispatches for the Queen. (See Plantations of Ireland and Tudor conquest of Ireland). At that point he took the opportunity of presenting the Queen with his "A Discourse of a Discoverie for a New Passage to Cataia" (Cathay) (published in revised form in 1576), treating of the exploration of a Northwest Passage by America to China. Within the year he had set down an account of his strange and turbulent visions, in which he received the homage of Solomon and Job, and their promise to grant him access to secret mystical knowledge.