Gregor has been like a physical insect for a long time in that he has been toiling in a dehumanizing job that he hates and is ill-suited for.
Gregor is a traveling salesman. As he lies in bed the morning he turns into an insect, he thinks about all that he dislikes about his job: the endless traveling, the bad food, and his anxieties about train schedules. Further, because he travels so much, he never gets to put down roots and get to know a specific set of people. Because he has to get up so early in the morning, he is chronically exhausted. He life is consumed with work that brings him no satisfaction.
Gregor works for a demanding boss but is afraid to say anything to him, because he needs the job to pay off debts his parents owe to this boss.
Gregor's relationship with his family also highlights his dehumanization. Although he seems to love them, they see him as valuable not for himself, but for the paycheck he brings in. Once he can no longer work, they become more and more resentful of having to provide for him. His parents were also quite willing for him to work at a job he hated, because it served their own needs.
Being only valued for the money he earns either for his company or his family reduces Gregor to level of something like an insect, so it is fitting that he comes to resemble externally what he feels like internally.