Do the usurious gains that were given out with the intention of purifying one’s wealth after repenting from them count as charity for which one is rewarded, or is there no reward for them, especially after considering the verse: "So whoever receives an admonition from his Lord and desists, then he may retain what has already passed?"

You have done well in repenting and striving to purify your wealth. Some scholars have mentioned that whoever entered into an interest-based transaction ignorantly of its prohibition is not obligated to dispose of the money they received before becoming aware of the prohibition. You will be rewarded, Allah willing, for spending these funds on charitable causes, as Ibn Taymiyyah said regarding disposing of ill-gotten wealth: "As for giving it away as charity, believing that it is not permissible for him to give it as charity, he will be rewarded for that."