How can God's command to Christians be upright in ruling by the Gospel, even though their religion has been abrogated by Islam and the Gospel has been distorted?

The text of the verse, "And let the People of the Gospel judge by what Allah has revealed therein. And whoever does not judge by what Allah has revealed - then it is those who are the defiantly disobedient," means what was in effect before the Gospel was abrogated by the Qur'an. However, after the abrogation, judgment is to be made only by the Qur'an, because it abrogates all revealed books.