Even assuming that the dates given in the Samguk Sagi are accurate, Baekje through Gusu's reign was probably a confederacy, not yet a kingdom.
In 216, he prevailed against the Mohe forces at Sado Castle, and in 222, he destroyed a 5,000-strong force of Silla, the rival Korean kingdom. However, the Samguk Sagi records many Baekje defeats against the Mohe and Silla, as well as natural disasters towards the end of Gusu's reign.
Upon Gusu's death, his eldest son Saban briefly became the 7th king. Saban was dethroned by Goi, reported as Chogo's brother in the Samguk Sagi, but believed to be of a rival royal line by modern scholars. Indicating continuing rivalry between the two lines, Gusu's second son became the 11th king Biryu, and the name of the 14th king Geungusu indicates descent from the Gusu line.