According to Wikipedia, gyōza were introduced to Japan after World War II.
The Japanese version developed from jiaozi recipes brought back by Japanese colonizers returning from Manchukuo, the Japanese-backed state in Northeast China. After the war, many repatriates opened yatai, or food stalls, where they served familiar Chinese-inspired foods, including gyōza alongside ramen. This helped turn the dumpling into a widespread Japanese dish.
Gyōza are related to Chinese jiaozi, but Japanese preparation developed its own recognizable character. Japanese versions commonly use a thinner wrapper and a filling rich in garlic, pork, cabbage, and Asian chives. The most familiar style is yaki-gyōza: one side is fried until crisp, then water is added and the pan is covered so the upper portion steams.
The conflict named in the answer is World War II, not the First Sino-Japanese War, the Russo-Japanese War, or the Korean War. The postwar setting matters because returning settlers and the food stalls they established were central to gyōza’s popularization in Japan.