There are two accounts explaining how Kenneth Hale became the Gorilla Man. The earliest account suggested that Hale was plagued by nightmares of a Gorilla Man in deepest Africa. Becoming obsessed with finding the Gorilla Man, Hale traveled to Africa where he tracked down the mythic beast and fought it to death. In killing the Gorilla Man, Ken Hale himself was transformed into a new Gorilla Man.
In the revised version of the story, it's stated Kenneth instead sought immortality and killed a gorilla that was supposedly immortal. However, upon death, the gorilla turned into a normal human, and Kenneth found that while he was now immortal, he had become a gorilla himself.