The word Charity really has connotations that should be deserted when trying to understand this last of Lewis’ loves. Typically, I have heard this love referred to as agape, meaning “love”–specifically referring to the love of God for His people. This is how we are to love God in return. This is also how we are to love those next to us, and the gifts bestowed upon us.
But Divine Gift-love in the man enables him to love what is not naturally lovable; lepers, criminals, enemies, morons, the sulky, the superior, and the sneering. Finally, by a high paradox, God enables men to have a Gift-love towards Himself. (p. 322)
The way we have a Gift-love for God is to give Him ourselves.