Geungusu came to the throne in 375 to become the fourteenth ruler of the Baekje kingdom. While he was still the crown prince, he distinguished himself in many battles led by his father, King Geunchogo. In 369, for instance, when King Gogukwon from the northern kingdom of Goguryeo led 20-thousand troops himself to invade Baekje, the crown prince of Baekje fought against them to win the battle, capturing 5,000 prisoners. Two years later, he led 30-thousand troops, along with his father, to attack Pyongyang Castle of Goguryeo. At the time, he drew his arrow and fatally wounded the Goguryeo king, who eventually died.

Due to Baekje’s attack on Pyongyang Castle and the killing of the Goguryeo monarch during his father’s reign, King Geungusu also had to deal with hostile relations with the northern neighbor. Moreover, his predecessor had expanded the territory into the present-day Hwanghae Province in western North Korea, which had initially belonged to Goguryeo. As anyone could imagine, Goguryeo tried hard to reclaim the lost area. In fact, Goguryeo repeatedly invaded Baekje during King Geungusu’s reign to recover the Han River area. In response to Goguryeo’s invasion in 375 and 376, the king led 30-thousand soldiers to attack Pyongyang Castle again in 377, this time as king. In retaliation, Goguryeo later invaded Baekje again.

In response to Goguryeo forming an alliance with the southeastern kingdom of Silla to keep Baekje in check, King Geungusu continued his father’s policy of maintaining friendly relations with the Jin Dynasty of China and Yamato’s Japan.

Domestically, the king had to cope with a number of natural disasters. According to The History of Three Kingdoms, mud rain poured down all day long on a summer day in 379. The following year, an epidemic broke out, claiming many lives. Also, the ground cracked in the summer of the same year. In 382, a long drought and famine gripped the country, with some starving people even selling their own children. The king opened the state storage and distributed grains to the people to feed them. In 384, the year when the king died, a big tree in the yard of the royal palace suddenly fell.

Despite various calamities, the Baekje kingdom maintained its strong military and economic power during the king’s reign. In other words, King Geungusu successfully inherited and kept his father’s legacy. He died in 384 after nine years on the throne. He was succeeded by his eldest son, and his second son also became king later.