This is a good question. There are several main themes to the work, The Metamorphoses. In light of this, let me name a few of them.
First, there is the theme of freedom and freewill. Gregor from the beginning of the work did not really exercise his freewill and freedom. He just took things as they came. Even when he turn into a bug, he just accepted his fate without a fight. Finally, he died. His life in some sense was tragic and meaningless.
Another theme is the idea of duty. Gregor is working hard for a good reason. He is the bread winner of the family. When he experiences his unfortunate transformation there is a sense of duty that the family has for him, which fades. For example, Grete says:
He must go, "cried Gregor’s sister, “that’s the only solution, Father. You must just try to get rid of the idea that this is Gregor. The fact that we’ve believed it for so long is the root of all our trouble."
Therefore, the question is to what extent does our duty go? There is no resolution. So, on the one hand, a person needs to be free, but on the other hand there is an important sense of duty.