There are many things in the Gospel that would firmly disagree with Islam. The divinity of Christ, the Fatherhood of God, God's love for sinners, the death and resurrection of Jesus and many other things. When a Muslim tells you not to believe those things, they are telling you not to judge by the Gospel. So you take them to Qur'an 5:47 and show them that their own Holy book tells us, as Christians, to judge by the Gospel, as it is the revealed word of Allah. If we don't, the same verse says we're rebels.
The immediate first response is usually "your Bible's been corrupted!" The temptation for many Christians here is to go back through history to demonstrate that the Bible has been reliably preserved, and that even if we only go back to 500 A.D., we still have the same Bible today that Muhammad had in 600 A.D. But since we're looking at the Qur'an, we don't actually have to do that. All we have to do is flip a few pages to Qur'an 18:27 where we're told that none can alter Allah's words. The apostles couldn't change Allah's words. The Church "fathers" couldn't alter Allah's words. The Reformers couldn't alter God's words. King James could not alter God's words. One thing common to both Islam and Christianity is that God is so powerful that even though lots of false prophets go out, God's word remains in existence in all its glory.
But this leaves Muslims with a dilemma. If the Gospel is the authoritative, revealed word of God, and none can alter God's words, that means that everything in the Gospel must be true. But if everything in the Gospel is true, Islam is contradictory. Therefore, if the Gospel is true, Islam is false because the Gospel contradicts it. If the Gospel is false, Islam is false because it affirms the truth of the Gospel. In both scenarios, Islam loses.
So, I think it's time for Muslims to start thinking about that Gospel Muhammad seemed to love so much. What does God tell us in the scriptures which came before Muhammad, which the Qur'an clearly tells us are 100% true, and no one can alter? My Muslim friends, it tells us that God is our Heavenly Father, that Jesus is divine, that Jesus did die on the cross and rise again etc. All of these things, the Qur'an denies, but if you believe these things, you can be saved. In fact, Romans 10:9 tells us that if you confess with your mouth that Jesus is Lord, and believe in your heart that God raised Him from the dead, you will be saved. So do what Muhammad told you, and judge by the Gospel. Do not be a transgressor by rejecting it. Repent and be saved.