In The Metamorphosis, how does Gregor feel about his job and how do you know?

Gregor dislikes his job with an intensity that appears nearly obsessive. He reflects on his feelings about his job, his boss and his co-workers early in the story when he awakes in bed. 

Gregor's first thoughts about the hassles presented by the travel required in his job end with the phrase, "To the devil with it all!" Indeed, Gregor's transformation now makes working impossible. However, Gregor's whole sense of self and sense of normalcy is wrapped up in his bitter position at work. Despite his new condition, he tries to go to work, worrying over the prospect of missing a day. 

The breach between Gregor's fantasies of quitting and his attachment to his normal work-life is never explicitly articulated, but the ineffectual and self-pitying Gregor certainly dreams of resigning and telling off his boss. 

"If I didn't hold back for my parents' sake, I'd have quit ages ago. I would go to the boss and state my opinion out loud from the bottom of my heart. He would've fallen right off his desk!"

Consoling himself with thoughts of a future where he will no longer work as a traveling salesman, Gregor uses the consolation as a way to convince himself to get up and go to work and face another day.

The debt that his family owes provides one formal reason that Gregor feels he cannot quit the job he dislikes so much, but his reflections also suggest that Gregor fears his boss as much as he despises him. Again, this dynamic is only implied in the text, although Gregor's passive fantasizing should be familiar to readers as a common mode of powerless and internal resistance to economic pressures on the lower and middle classes. Gregor and his fantasy, in other words, fit neatly into a typical category of class-based resentment as rendered in literature. 

This information, both that which is overtly stated and that which is implied, in delivered via Gregor's quoted thought and un-quoted material intended to represent his thinking. The narration utilizes a degree of omnipotence in this regard. Situated as a narrative voice outside of the character, the narrative voice nonetheless has full access to Gregor's thoughts and feelings.