Is a son residing in the UAE considered undutiful if he refuses to repay his father's usurious loan in Gaza, given his financial inability and personal debts, even though his father is threatened with imprisonment?

Your father's action of borrowing with interest is a grave mistake, but that does not nullify his right to your dutifulness and kindness. You are not obligated to pay off your father's debt if you do not have surplus money beyond what you need for your essential needs. However, you must be dutiful and kind to him with all that you are able, and frequently supplicate for him.