Is ghusl and repentance obligatory for one who has become discontent and distressed by life's affairs and felt dissatisfied with God's decree, even accusing God of injustice in his heart, though he did not utter it, while believing that he is among the people of Hellfire?

People have different levels in dealing with painful decrees (qadar): the highest is contentment (rida) with increased love and yearning for Allah, followed by the level of gratitude for them, then the level of patience (sabr), falling below which into discontent (tasakhkhut) is considered a deficiency in faith.

The discontented person is sinful and must repent and exhibit beautiful patience. However, mere inner thoughts are not held against a person as long as they do not settle in the heart and become firm convictions; as the Prophet (peace be upon him) said: "Indeed, Allah has pardoned my Ummah for whatever thoughts occur to their minds, as long as they do not act upon them or speak about them." You are not required to perform ghusl or anything else. This feeling you have might be a waswas (whisper from Satan), so you should seek refuge from it and disregard it.