Gregor's father doesn't work while Gregor is working as a travelling salesman, he spends all day reading the papers out loud and enjoying his retirement. Sure, he is an older man and presumably worked hard all his life, but once Gregor morphs, his father goes back to work. He was not just living off the success of his son, which would be one thing, but he tied a harness to his son's neck and was forcing him to pull the plow. It seems Gregor was having a successful career in the army, a rank of lieutenant is a big deal, but because of his father's mistake he is recalled home. His father is being selfish, by screwing up his business and then forcing his son to work off his debt, he has potentially altered the course of the family. Gregor could have very well risen up the ranks to become a respected and well paid high ranking officer, making the Samsa name famous. Instead he is a middling employee who becomes an insect.
The most despicable thing about Gregor's father is that, besides the fact that he doesn't own his sins, he disowns his son instead. Look at part one where for the majority of the time he believes Gregor to be still human. He completely throws his son under the bus for being late, he doesn't stall the manager, he doesn't try to defend his son, he just capitulates to the boss. We see later he defends his wife and daughter from the perceived threat of Gregor, yet he has no inclination whatsoever to defend his son who is keeping him out of debtor's prison and his entire family in a nice apartment. The only actions that Gregor's father makes, that are described in any sort of depth in the entire novella, are the few times he attacks his son. The apple assault helps lead to Gregor's death, yet he suffers no grief for his son's death (or at least disappearance if we want to quibble with how much of Gregor was left in his insect body by the end). Gregor's father is the kind of man that Edward R. Murrow hated most. He is a weak man who allows injustice to happen in front of him because he doesn't have the nerve to say anything, and relies on others to clean up his own mess.