How does Kafka use Gregor's metamorphosis into an insect as a metaphor? What does he seem to be suggesting about the human condition by using this particular image?
The comment on the human condition offered in Gregor's figure seems to relate to family life (and the tendency toward exploitation within that construct) and to work life (and the tendency toward exploitation and indignity in that construct).
However, Kafka's story "The Metamorphosis" is known in part for being open to interpretation.
"I cannot make you understand. I cannot make anyone understand what is happening inside me. I cannot even explain it to myself."
The narrative itself does not strongly indicate a single meaning behind Gregor's transformation into an insect and instead offers hints that leave multiple interpretations available to the reader.
Gregor's family relies on him to bring in income. His father has been disgraced and his sister does not yet work. The family conducts its life on the assumption that Gregor will get up every day and go to a job that makes him bitter and spiteful, a job that, in a way, dehumanizes him.
The circumstances of Gregor's life take away his chances at achieving some dignity. He has no life of his own and he receives no respect for the sacrifice of his autonomy or for his toil. Rather than being an autonomous adult, imbued with agency and self-respect, Gregor is an automaton, expected to thanklessly obey orders and work for others. This may be the central comment in the metaphor of Gregor's transformation.
Also, Gregor's change is not noticed right away. His loss of humanity is, it would seem, already a foregone conclusion as the story opens. Thus, there is a way of looking at the narrative that suggests Gregor has not transformed within the story (or even just before the story begins) but rather he has long ago lost his dignity and humanity.
Seen in this light, the metamorphosis of the narrative is not Gregor's but can be attributed to his family. They come to realize that Gregor has been reduced to a state of paralysis (due in large part to his own bitterness) and they find that it is now necessary to make adjustments. Their worker bee (or beetle) can no longer be counted on to do their work for them.
This approach does require something of a leap, but it does lead to a broad view of the idea of Gregor as a metaphorical figure symbolizing a personal psychological and emotional response to a situation of indignity and exploitation. Perhaps he has finally come to see himself as others have learned to see him.
“He was a tool of the boss, without brains or backbone.”
Gregor's reflections are bitter and spiteful when he explores his feelings on work and family obligations. He longs to shout down his bosses and to quit his job. He dreams of communicating his feelings fully and loudly.
“If I didn't have my parents to think about I'd have given in my notice a long time ago, I'd have gone up to the boss and told him just what I think, tell him everything I would, let him know just what I feel. He'd fall right off his desk!"
He despairs in his situation. His lack of speech as an insect is consistent with his sense of being silenced and marginalized in his job and family.
We might see Gregor's metamorphosis as an insect as a straightforward metaphor of self-realization.
In these various ways of reading Gregor's transformation into an insect, there is a suggestion of institutional numbness or institutional indifference. Gregor's family does not despise him -- they simply forget to recognize his right to a life of his own with respect and freedom. They feel that he owes something to the family (the institution) and what he owes amounts to a life of toil and tedium.
Gregor's bosses are not necessarily bad people. Rather, they represent and unfeeling corporate/commercial point of view that does not value a person's inner life. Work, as an institution, can take away an individual's sense of personal dignity and value.
What does this say about the human condition? There may be a comment here implying that there are dangers in relying too heavily on institutions, however traditional, to make up one's life and the meaning of that life.