'Gain' was a British special forces operation by D Squadron, 1^st Special Air Service, to cut rail communications in the Orléans region to the south-west of Paris in German-occupied France in an effort to prevent German forces from other parts of France from reaching the Normandy area in substantial numbers (16 June/mid-August 1944).

Under the command of Major Ian Fenwick, the squadron was parachuted into its operational area in several parties, starting with a small group that arrived in the Forêt de Fontainebleu to establish contact with the local resistance forces and to find the right location for the required SAS base. The first attacks, including two successful forays against the road connecting Orléans and Pithiviers, were undertaken on foot, but the SAS squadron then received a number of Jeeps that allowed it to move farther and faster with heavier weapons and greater quantities of explosives. Even so, the operation was not as successful as it might have been, for the SAS base was changed often and disruptively for fear of traitors in the ranks of the local resistance forces.

Several SAS groups also ran into German ambushes, and on 6 August the Germans discovered and attacked the squadron’s current base, in the process killing Fenwick during a firefight in the village of Chambon as he and a number of his troopers sought to escape in a Jeep. The rest of the squadron continued the operation to the middle of August, when the surviving troopers linked up with the advancing US forces at the end of a highly successful effort that had inflicted many casualties and substantial damage on the Germans.