Gaō was the ancestor of Reima and leader of a pack of 300 wolves.
Ginga Nagareboshi Gin
Gaō lived in Hokkaido around 100 years ago, during the Meiji period, when humans started to threaten the wolves and take away their territory. They had little food and some of Gaō's minions even killed domestic animals and humans and got shot.
Gaō made a hard decision and eventually lead his pack into a huge underworld cave known as the Underworld kingdom. He decided to make it their new home in order to save his species from extinction.
In the Underworld, Gaō and his warriors fought the serpent monster Jaryuuki. Many were killed but they eventually defeated the beast. Gaō became the king of the Underworld.
A bit later, his old enemy Black Wolf came to ask for help with his pack. The two packs had had fights earlier but Gaō let them stay since there was enough room and he knew how hard their situation was.
Black Wolf and his kin was evil and wanted to take over Gaō's pack. However, this didn't happen during Gaō's lifetime and it's unknown what eventually happened to him and if he ever knew that he had let in a dangerous enemy.