Here's a partial list of his books, many of which are out of print. I particularly like Smallbone Deceased, A Crack in the Teacup, Blood And Judgement and the short stories about Mr. Calder and Mr. Behrens: Game Without Rules and Mr. Calder and Mr. Behrens.
Two of Gilbert's most popular characters, the amiable and elderly spies Daniel John Calder and Samuel Behrens, featured in Game Without Rules (1967), a collection of short stories. As ruthless as they are polite, Calder and Behrens are given the tasks that are too ugly for their younger and more glamorous colleagues. When duty calls, they receive a coded bank statement ("your account is very seriously overdrawn… ") and are sent on discreet missions to assassinate enemy operatives.
More here and here.