When poor Gregor finally manages to open the door that he has had to struggle so much to open, the response of his parents is obviously one of great shock to their strangely tranformed son who went to bed a human and woke up the next day a disgusting insect. Note how his parents are shown to respond with the sight of their transformed son:
The mother, who, despite the office manager's presence, stood there with her hair still undone and bristling, first gaped at the father, clasping her hands, then took two steps toward Gregor and collapsed, her petticoats flouncing out all around her and her face sinking quite undetectably into her breasts. The father clenched his fist, glaring at Gregor as if trying to shove him back into his room, then peered unsteadily around the parlour before covering his eyes with his hands and weeping so hard that his powerful chest began to quake.
Note the way that the mother responds with shocked incredulity, apparently fainting with the surprise, whereas the initial reaction of his father is to respond with anger, before giving away to deeper emotions and beginning to cry. It is interesting that the response of the father foreshadows the later violence that he visits upon his son, whereas the helpless response of the mother clearly displays her inability to know how to cope with the situation. The father's response in particular shows the anger and seeds of hatred that exist in the relationship.