Some quotes that show Gregor's father has dehumanized him come from embedded flashback narration that provides (through indirect interior monologue) Gregor's memories of part of the family history. The family business had failed in the past and Gregor, with great pleasure, enthusiasm, and devotion, had at that time taken over all financial responsibility. After a while, as the indirect interior narration explains, his father's gratitude and loving appreciation faded to the point that Gregor's efforts and contributions were seen as routine, expected, and mechanical, so much so that affection was lost and Gregor's father stopped standing up to greet him during the few times he was home from traveling. Dehumanization is defined as the rendering of unique human qualities and actions as nothing more than routine and mechanical, as not human-like.
Gregor's only concern at that time had been to arrange things. . . [H]e started working especially hard, with a fiery vigour that raised him from a junior salesman to a travelling representative almost overnight. . . Gregor converted his success at work straight into cash that he could lay on the table at home. . . They had been good times and they had never come again, at least not with the same splendour. . . [The family] had even got used to it, both Gregor and the family. . . although there was no longer much warm affection given in return.
[W]hen Gregor came back from his business trips, [his father] would receive him sitting in the armchair in his nightgown when he came back in the evenings; [he] was hardly even able to stand up but, as a sign of his pleasure, would just raise his arms.
It can be argued that Gregor has been thoroughly dehumanized by his metamorphosis (his self has been dehumanized by an unknown mechanism or force). In this case, Gregor's father's reactions to his metamorphosed state are not evidence of his dehumanization of Gregor. The family, after all, hears only an animal voice. If the argument is being made that Gregor's father's post-metamorphosis actions further dehumanize Gregor, then a good quote relates to the father's initial violent reaction to the "monster" that Gregor has become, which is exhibited when he drives Gregor back into his room when Gregor impulsively goes after the fleeing chief clerk.
Gregor's father seized the chief clerk's stick in his right hand (the chief clerk had left it behind on a chair, along with his hat and overcoat), picked up a large newspaper from the table with his left, and used them to drive Gregor back into his room, stamping his foot at him as he went. . . Nothing would stop Gregor's father as he drove him back, making hissing noises at him like a wild man.
It might also be argued that the case for dehumanization Kafka is making in this story is that Gregor's employment—though productive of a rewarding income and sense of accomplishment—has dehumanized Gregor despite having given Gregor and his family, whom he loves very much, a pleasant and leisurely life to live (except for Gregor's arduous travel and demanding early hours). It can be argued that, in Kafka's eyes, though the rewards are great, the rewards do not balance the dehumanization caused by Gregor's employment conditions. Some of the conditions that dehumanize Gregor are the lonely, unpleasant hotel rooms; the office gossip that arises about him out of envy; the misconceptions the employer develops during Gregor's long absences resulting from traveling.
[I]t's very easy for a businessman like him to make mistakes about his employees. . . [W]e travellers spend almost the whole year away from the office, so that we can very easily fall victim to gossip and chance and groundless complaints, and it's almost impossible to defend yourself from that sort of thing, we don't usually even hear about them. . . we feel the harmful effects of what's been going on without even knowing what caused them.