Whenever we consider a work of literature as complex as "The Metamorphosis," it is clear that there are a variety of themes that we could draw upon. However, from my perspective, the three main themes are that of the nastiness of the world of work, betrayal and isolation and self-sacrifice.
Especially at the beginning of the story it is important to be aware of how much Gregor detests his job and is worn down by it. He bemoans the "strenuous" profession he works in, and the first thing he wants to do when he wakes up is go to sleep again to try and forget about "this nonsense." He talks about the conditions of his job and explains how hard they are. He is a man who is unfulfilled by his job and oppressed by it. Not only that, but it is clear that they do not trust him at his work, and his boss is quick to come and fire him.
Betrayal is a theme that is evident through the transformation not only in Gregor but in particular in his sister. We are told that it is she that Gregor loves. He is willing to work so hard to help look after her. Relationships between him and his father in particular are strained, but it is Gregor's sister that loves him and also tries to look after him at the beginning after his transformation. However, given this mutual affection, it is therefore a big surprise that at the end of the story it is his sister that persuades their parents to move on and pretending that Gregor is not actually Gregor but an insect that is trying to kick them out of their house. This betrayal suggests that even the closest of relationships cannot be trusted.
Lastly, ironically it appears that Gregor led quite an insect-like existence even before his transformation. He led a lonely, isolated life before his transformation, and this is continued afterwards, as he is unable to communicate with anyone as an insect. Note too the irony in that Gregor as an insect can only hear what others want and desire and cannot communicate himself. Before his transformation, he seemed to be a character who would always try and meet the needs of others rather than his own needs, so perhaps this is appropriate.
Overall, this story paints a bleak picture of human existence that is marked by isolation, alienation, dissatisfaction and betrayal.