When German troops invaded Norway on 9 April 1940, Gregers Gram became active in the resistance movement. After serving as a soldier in the early battles following the German invasion, he soon became involved in propaganda activity and the illegal press, he had to flee from Norway in 1941. After arriving in Great Britain, he was recruited by the Special Operations Executive and received military training with the Norwegian Independent Company 1. He was paradropped into Norway together with Max Manus on 12 March 1943, and the two performed a successful sabotage mission ("Operation Mardonius"). Gram and Manus left Norway after "Operation Mardonius", returning to the United Kingdom, where they stayed until October 1943. Gram was awarded the British Military Medal as a result of his participation in the operation, the recommendation for the award describes how Manus became ill with pneumonia shortly after they arrived in Norway, meaning that Gram had to undertake much of the organisation on the ground, and also nursing Manus during his recovery. The most successful attack was on 28 April 1943 which resulted in the sinking of two ships, and damage to a third, despite the fact it was a very light night, and water conditions were such that the canoes generated a very visible phosphorescence in their wake. Gram was also decorated with Norway's War Cross with sword in summer 1943, presented to him by King Haakon at a ceremony at the training school STS 26 in Scotland, near Nethy Bridge. In addition to Gram, Manus, and Rønneberg and Poulsson from the Rjukan missions, also received the War Cross with sword at this ceremony. Also present were Crown Prince Olav, Minister of Defence Oscar Torp, and Chief of Defence of Norway General von Tangen Hansteen.