Here "justified" refers to being made right with God. You can translate this in an active form. Alternate translation: "God makes them right with himself as a free gift, because Christ Jesus sets them free" (See: [[rc://en/ta/man/translate/figs-explicit]] and [[rc://en/ta/man/translate/figs-activepassive]])
This means that they are justified without having to earn or merit being justified. God freely justifies them. Alternate translation: "they are made right with God without earning it"