"G section" is fictitious, but is effectively the French section of the Special Operations Executive, the wartime unit set up by Churchill to promote resistance and sabotage behind enemy lines. The SOE brought together pre-existing units from MI6, the War Office and the Foreign Office to wage war "by other means". It was occasionally known as The Ministry of Ungentlemanly Warfare. Winston Churchill said the purpose and intent of the SOE was“to set Europe ablaze” and liberate the continent from German occupation.
The SOE operated right across occupied Europe, and enjoyed particular successes in Yugoslavia and Greece. The US Office of Strategic Services, forerunner of the CIA, was partly modelled on the SOE.
When Clement Attlee won the 1945 general election, he immediately shut down the SOE, eliminating a worldwide intelligence network of thousands of people. A handful were taken on by MI6; the rest had to return to peacetime roles.
SOE Memorial at Valençay, France