The following might help Muslims understand the nature of the Gospels better:

The 4 Gospels are the "ahaadith a-sahiiha" (the correct traditions) on the life of Jesus.

In the law courts at least two witnesses were needed to prove a truth. We have the three synoptic Gospels on the life of Jesus, Matthew, Mark, and Luke which testify to Jesus' life and sayings. The Gospel of John (injeel yuhana) gives the inner secrets (al-asraar) of Jesus for the believer.

John deals with Jesus' sayings concerning the Jewish holidays, the coming of the Holy Spirit and his resurrection sayings as the risen Christ.

Each Gospel has its purpose. All of them are written witnesses to the true Gospel (al-injeel as-sahih) which is Jesus Himself.