Sadahide's father, Takasato was a branch family of the Gamo clan, and Sadahide's cousin Hidenori GAMO had been taking over the reigns of head family, but while Hidenori was leaning toward the Muromachi bakufu (Japanese feudal government headed by a shogun), Takasato and Sadahide gained their lord Sadayori ROKKAKU's confidence, and he was given a word "Sada" with the backing of the lord, and identified himself as Sadahide, he attacked Hidenori in 1522, and destroyed Otowa-jo Castle which was the Gamo clan's castle at that time.
定秀の父・高郷は蒲生氏の分家で、本家の家督は定秀の従兄弟・蒲生秀紀が継いでいたが、秀紀が室町幕府寄りであったのに対し、高郷・定秀は主君・六角定頼の信任を受けていたため、その後ろ盾を得て「定」の一字を賜って定秀と名乗り、大永2年（1522年）に秀紀を攻撃し、当時の蒲生氏の居城であった音羽城を破却した。