Gook (グック, Gukku) is a Shinigami.
Gook's most prominent feature is his animal-like head, which is that of a cattle skull with long horns. He also wears a large, dark-green beaded necklace (red in the manga) and has yellow eyes.
Like Deridovely, Gook enjoys gambling, although he is rather poor at it. He is also very lazy.
Gook appears gambling with Deridovely in the Shinigami Realm in the first episode of the anime and witnesses Ryuk departing to the Human World. Gook only appears again in Ryuk's flashbacks, in which he explains the Shinigami Realm to Light.
Gook also appears in the prologue chapter of the Death Note One-Shot Special. He thinks that Ryuk is in the Human World but is mistaken, as it is Midora who is in the Human World. He chats with Ryuk about the apples that Midora brought from the Human World for the Shinigami King in exchange for a Death Note.
Takeshi Obata said that, as Gook appears in the first chapter, he wanted for him to look like a monster to keep it simple. Obata decided to use a cattle skull in the design, as he felt that using a regular skull would be boring.