AFAIK, Unreal engine can force gamma through GPU drivers. Still, to implement proper dark nights, a huge work in level design is needed. Many textures need correction, night light sources need to be corrected. Old dark nights led to areas that were fairly illuminated, while there were areas that were pitch black, due to texture coloring and lighting. Overall, it did not look good enough, e.g. there were grass fields that were shining during the night on a pitch black ground.