Qur'an 7:157—Those who follow the Messenger, the Prophet who can neither read nor write, whom they find written with them in the Taurat (Torah) and the Injeel (Gospel) . . . it is they who will be successful.
The Qur'an also suggests that the Gospel was given to Jesus:
Qur'an 5:46—And in their footsteps We sent Jesus the son of Mary, confirming the Law that had come before him: We sent him the Gospel: therein was guidance and light, and confirmation of the Law that had come before him: a guidance and an admonition to those who fear God.
Since we have no record of any book given to Jesus, Muslims have a problem, for the Qur'an affirms that Christians have access to the Gospel and that we must judge by what it says:
Qur'an 5:47—Let the people of the Gospel judge by what Allah hath revealed therein. If any do fail to judge by (the light of) what Allah hath revealed, they are (no better than) those who rebel.
Qur'an 5:68—Say: "O People of the Book! ye have no ground to stand upon unless ye stand fast by the Law, the Gospel, and all the revelation that has come to you from your Lord."
How can we obey a book given to Jesus when we have no record of any book given to Jesus? The only way to reconcile (1) the fact that we have no book from Jesus, and (2) the fact that the Qur'an affirms some book in our possession, is to adopt the Christian perspective of the Gospel. The Gospel was a message (the "Good News") delivered by Jesus to his followers and then proclaimed by these followers. This message was recorded in books called "Gospels" (because they contain the Good News). These books were eventually collected into a larger book called "The Gospel."
From the second century on, the four Gospels (Matthew, Mark, Luke, and John) were known as "The Gospel" or "The Four-Fold Gospel." But the core message of the four Gospels is that Jesus is the divine Son of God (Matthew 3:17), who died on the cross for our sins (Mark 10:45) and rose from the dead (Matthew 28:5-7). Islam, by contrast, teaches that Jesus was only a prophet (Qur'an 4:171), who never died (Qur'an 4:157) and didn't rise from the dead (Qur'an 4:158). Because the Qur'an commands us to judge by the Gospel, and the only Gospel we have access to is the book containing the four Gospels, we have no choice but to reject Islam, which contradicts the message of the Gospel.
To put the matter more succinctly, if the Gospel is the inspired, preserved, authoritative Word of God, then Islam is false, because Islam contradicts the Gospel. If, on the other hand, the Gospel is not the inspired, preserved, authoritative Word of God, then Islam is false, because Islam affirms the inspiration, preservation, and authority of the Gospel.
Either way, Islam is false. Muslims have no rational choice but to reject Islam and search for the truth.
Here's N. T. Wright on the Gospel message: