The word "horrible" suggests that Gregor's new appearance is absolutely revolting, as he has converted into an ugly bug.
Gregor is the protagonist of the story, he is about 23 years old. He works as a commercial traveler to support his sister and his parents. He awakens one morning like a monstrous insect. After the metamorphosis, Gregor is unable to work, and this will force his father, his mother and his sister, to work to sustain themselves. He spends most of his time in his room and witnesses the abandonment and disdain of his family, which is growing little by little. Sometimes he leaves his room to tour the house in secret.