There are many different themes related to this story, but I think the most developed one is that of alienation. Gregor's transformation into a bug is a symbol of how he feels as a human - alienated and separated from society. He has a job he doesn't like, a boss he doesn't like, a family he doesn't bond with, and no social life at all.
His relationship with his father is one of subservience. As a bug, the few times that Gregor tries to escape the room, his father forces him back.
- With his left hand, his father picked up a large newspaper from the table and, stamping his feet on the floor, he set out to drive Gregor back into his room by waving the cane and the newspaper.
This is meant to be symbolic of how his father treats him as a human - with fierceness, pushing Gregor away. Gregor's mother seems to care for him, but not so much that she can overcome her revulsion and care for him. Only Greta will care for Gregor. But even in this relationship, there is a lack of understanding. Gregor thinks he is close to his sister, but he misunderstands her. He believes music is her passion, but she is not very good and does not want to pursue it, as he assumes she does.
Being a bug is an outward manifestation of the situation that already existed, and it brings the situation to a close. Gregor's alienation from his family and society is his downfall in the end.