Is the questioner's lack of knowledge regarding the punishment for backbiting (gheeba) and its status as a major sin considered a mistake, given their repentance and regret for backbiting about living and deceased individuals?

Backbiting is a major sin, and Allah has forbidden it in the Noble Qur’an and through the words of His Messenger. If you regret and repent from this sin, you must seek forgiveness from the person you backbit about if they are alive. If you are unable to seek their forgiveness due to their death or fear of harm, then you must supplicate for them and mention their good qualities in the same places where you backbit about them, while increasing in good deeds that expiate sins and drawing closer to Allah. The fact that you did not know it was a major sin does not absolve you of its consequences.