This entire story revolves around conflicts between Gregor and his environment.
In Gregor vs nature, the primary example is when Gregor is turned into a giant cockroach-like insect. Besides his appearance, human food stops appealing to him, he cannot sleep unless hiding, he cannot maneuver doors and door knobs, and his ability to speak disappears.
Gregor vs self is seen as he battles with himself to continue working at a job he hates. His hours are long, he travels a great deal, and he is unable to have much of a personal life. Yet he continues to go to work because his income is needed at home. His personal sense of alienation from the family may be symbolized in his transition into a disgusting creature who does not belong.
The final conflict, and the most daunting for the character of Gregor (I think) is man vs man (person). Gregor's changed appearance disgusts everyone: his boss flees the apartment, his father tries to kill him, his sister ultimately rejects him (as do the tenants), his mother cannot bear to look at him, and even the cleaning lady becomes abusive.
Another form of this kind of conflict is the change that Gregor sees in his family. As long as he is able to provide financially for a family, none of whom work, Gregor is tolerated—even before his transformation. His family would probably find a way to live with him as a bug if he could still go to work and pay for them to live their lives of leisure.
Once Gregor is of no value to the others, he is longer accepted by them, and his complete sense of worthlessness (man vs. self) drives Gregor to stop eating, and in essence, commit suicide and die.