Botting was born in Oakley House near Royal Air Force Station Abingdon (RAF Abingdon) at Frilford Heath near Oxford, England on 19 July 1943. He was christened in the Church of England Parish Church of St. James the Great in Radley, Berkshire. His father, Pilot Officer Norman Arthur Botting DFC, a Dam Buster with 617 Squadron, was killed in action over Germany on 15 September 1943 when Gary was less than two months old—on his older sister Mavis' second birthday. Following the war, their mother Joan, a teacher, took up residence with Group Captain Leonard Cheshire VC, the father of their younger sister, Elizabeth, at Gumley Hall near Bedford Gardens, Market Harborough, Leicestershire and later she and the children moved with Cheshire to LeCourt, the name of the mansion he had acquired from his aunt in Hampshire. After witnessing the bombing of Nagasaki at the end of World War II, Cheshire, who had been raised high Anglican, began to examine various religions. Joan and he agreed about the nature of God as a person. Joan was baptized as a Jehovah's Witness in September 1948 and expected Cheshire to follow; when he converted to Roman Catholicism later that year instead, she moved with the children back to Radley.