How can the Islamic belief that the Gospel was revealed to Jesus (peace be upon him) be reconciled with the Christian narrative that it was written by the disciples of Christ after his ascension?

There is no contradiction between the two matters. First: Belief in the Injil revealed to Jesus, peace be upon him, which is one of the fundamentals of faith. Second: The Gospels existing today are not the original Injil revealed by God. Rather, they are histories authored by different men at different times, and they were not written by Christ. Furthermore, they have been subjected to alteration, loss, and multiple translations, which has deprived them of their original form.