Symbols in The Metamorphosis
At his most vulnerable, Gregor looks out of the window at what could be his salvation: a hospital. That it's so near and that he can't reach it symbolizes the futility of his situation, which, like the hospital, seems endless, a kind gray, lifeless box that's trapped him (perhaps forever) in this bug-like body.
Recall that in Part I, Kafka described a picture of Gregor from his time in the military, in which he was a lieutenant with a sharp uniform and a demanding presence, as if he deserved respect. Here, the uniform is on Gregor's father, which returns him to the position of respect and authority that he enjoyed before the collapse of his business. Kafka uses the symbol of the uniform to track the shift of power within the family.
Of all the items in the room, Gregor chooses to save the only one he made himself. In this way, the picture becomes of a symbol not just of his human desires and sexuality but of his self-succiency, which can, thus, be understood as his interiority and as his private emotional life. Without the picture, Gregor implies, all evidence of that life will cease to exist, and it will be as if he was never a person at all.
This is the seventh time that the word "door" (German: Tür) has been used in this short paragraph. This repetition makes the door a symbol of Gregor's inability to interact and communicate with the rest of his family and, by extension, all of humanity. This builds on the theme of isolation prevalent in the book.
Recall that when Grete was playing the violin earlier the parents also took their own sides, flanking her as she played. Here, Kafka implies that, because they each have their own side, they're divided in their thoughts and feelings about Gregor, as evidenced by the fact that the mother wanted to clean his room and couldn't imagine getting rid of him.
This dust is a symbol of Gregor's insignificance, as he's forgotten and pushed aside by his family, relegated to this room where dust cakes on his body, as if he's an old, unwanted piece of furniture. It's worth noting that dust, in itself, is composed of shed skin and particles that collect in the air, and that the dust in this room was likely created by Gregor himself.
In the course of the novel, Gregor's room has gone from a place that was devoid of privacy to one of illness and isolation and now, finally, to a trash dump where unwanted things are thrown in and forgotten (the implication here being that Gregor, too, is unwanted). Inevitably, of course, the trash will have to be taken out, and Gregor along with it. In this way, the state of the room becomes a symbol of Gregor's life.