During the 1939–1944 Nazi–Soviet occupation of Poland in the course of the World War II, Grzegorczyk was a voluntary member of the Home Army, the dominant Polish resistance movement formed in February 1942 by the Union of Armed Struggle and other Polish partisans with allegiance to the Polish Government-in-Exile as the armed wing of the Polish Underground State. As an insurgent of the Warsaw uprising under the pseudonym "Bułka", he provided a voluntary military service in the rank of Shooter in the 2nd Platoon of the Company '"Harcerska"' of the AK Battalion 'Gustaw' of the '"Róg"' Group of the Group "'Północ"' of the Home Army. As a result of Sonderkraftfahrzeug 301 Ausführung C Schwerer Ladungsträger Borgward B IV of the Panzer Abteilung (Funklenk) 302 on 13 August 1944 at the Jan Kiliński street in the Warsaw Old Town, which took life of more than 300 Polish partisans and civilians, he was injured in legs and placed in a field hospital. Several days later he rejoined the division, but by the health condition he did not participate in the further fights and was forced to leave Warsaw on 31 August 1944 as a civilian in the evacuation by the city canals from the Warsaw Old Town to the Warsaw Śródmieście.