Is it permissible for me to keep gifts and belongings from a person who passed away, with whom I had a friendship and a promise of marriage without my family's knowledge, and would donating these gifts alleviate his reckoning?

You committed a sin by establishing a friendship with a non-mahram man, and repentance is obligatory upon you. As for the gifts you took from him, they are ill-gotten gains (suht) and it is not permissible to own them; rather, they belong to his heirs. This is because most scholars believe that a gift to a fiancée is not valid unless it is given after the marriage, so it applies even more so to what is given before the engagement. The Hanafis have two opinions regarding the fiancé's gift: the first is that it is permissible to reclaim it absolutely, considering it a bribe; and the second is that it is permissible to reclaim it if the woman refuses to marry.