Muslim asked: “Why is Jesus Christ only a God in John’s Gospel?”
Christian response,

Matthew initiates his Gospel declaring Jesus as God as well as do the words and works of Jesus documented in all four-Gospels, but John’s Gospel is unique and set-apart from the synoptic Gospels as John focuses on Messiah’s deity, Matthew focuses on Messiah’s Jewishness in the flesh; Mark focuses on Messiah’s miracles; Luke focuses on Messiah’s birth and healing from a physician’s perspective.
The Holy Spirit has coordinated this so that mankind can understand God and internalize God’s purposes in Time and Eternity through the body of Jesus and His ministry upon this Earth. Jesus is Elohim (Genesis 1:1) and John emphasises this in the introductory paragraph of his Gospel as Jesus is the Living Word of God who became flesh and dwelt among us (John 1:1-14).
Unless Jesus is your Lord as Mediator for your sins, you will die in sin and not enter the Kingdom of God (John 8:24). Jesus is the ONLY WAY to life in Eternity (John 14:6).