If a person I've wronged forgives me without their knowledge of what I did, will the retribution on the Day of Judgment be waived from me after my repentance?

There are severe warnings and threats for those who harm Muslims. Allah Almighty says: "And those who harm believing men and believing women for [something] other than what they have earned have certainly borne slander and manifest sin." Therefore, one must be cautious and not revert to such an act. If you seek forgiveness from someone you have harmed through backbiting or the like, and they forgive you without you mentioning the backbiting, then that is sufficient for you and absolves you of the sin.