In "Tezuka Osamu Story: I Am Son-Goku," the demon god Gyu-mao serves as the primary antagonist during the science fiction segment set on Planet Sapphire in 3010. This narrative reimagines Journey to the West within a futuristic galactic context. Gyu-mao opposes the group led by the space-faring Buddhist monk Sanzo, which includes the extraterrestrial monkey girl Rular and the reimagined companions Hakkai and Sagojo.

Gyu-mao functions as a destructive force terrorizing the planet's peaceful inhabitants. His motivations embody chaos and resistance against Sanzo's mission. The character leverages Sapphire's unique alien environment, utilizing its flora and fauna in battles against the protagonists, blending traditional demonic traits with science fiction elements.

The climactic confrontation features an explosive battle where Gyu-mao demonstrates formidable power, nearly overwhelming the heroes and incapacitating Goku. Gyu-mao's defeat incorporates a tragic element of self-sacrifice by other characters, aligning with Buddhist themes of karma and redemption present in Tezuka's narrative approach. This conclusion emphasizes consequences over personal downfall.

Throughout the segment, Gyu-mao remains a symbolic obstacle challenging the protagonists, illustrating the story's thematic focus on sacrifice and moral lessons.