Gaki is Akakabuto's substitute and one of his sons.
Gaki looks exactly like his father, having a red-furred back and missing his right eye. He is also very large.
Ginga Nagareboshi Gin
Gaki's duty is to act like Akakabuto and make enemies think he actually is his father. His eye was blinded in order to make him look even more like his father.
When Riki tries to make an ambush on Akakabuto, the bear he attacks is actually Gaki. During the fight, Gaki loses three fingers as Riki cuts them off with a Light Attack. Akakabuto's brown bear servants appear and bury wounded Riki with trees. They mock and scold at Gaki due to his failed fighting and bossy attitude.
Then, Gaki is called by Akakabuto and informs that Riki is dead. After that, Gaki is not seen anymore.
Later, when the Ohu Army attacked the stronghold, Akakabuto's minions surprised the dogs. Gaki was likely one of the bears and was probably killed by the Ohu Army if he was.
See more pictures in Gaki/Gallery