The gospel refers to the good news of Jesus Christ. The good news is that anyone who believes that Jesus Christ died and rose on the third day can inherit eternal life in the kingdom of heaven (Romans 10:9-13).
The gospel books in the bible comprises the first four books in the New Testament including Matthew, Mark, Luke and John. The gospel books provide narrations about the life of Jesus Christ, from the time he was reincarnated into the world through the virgin Mary to the time he was betrayed by the Jews and crucified on the cross at the Calvary. They provide some similar contents that are parallel to each other, which includes the teachings and parables of Jesus Christ as well as the miracles he performed in Israel during his lifetime.
The message of the gospel is simple. God created human beings to be with him. The first human beings that God created is Adam and Eve. They sinned by letting themselves be tempted by the devil, who told them nothing will happen if they eat from the Tree of the Knowledge of Good and Evil. After they ate, their spiritual eyes are opened and they became ashamed by their naked bodies because they now know how to discern between right and wrong. It is a sin that they ate from the Tree of Knowledge of Good and Evil because God's commandment to them is to not eat from any tree in the middle of the Garden of Eden. There were only 2 trees in the Garden of Eden including the Tree of Life and the Tree of Knowledge of Good and Evil (Genesis 3).
Sin creates a wall between us and God so that we can no longer approach him face to face (Isaiah 59:2). As a result, all humans will go to hell except the Israelites who is God's people. This is the law of the Old Testament. Later, Jesus Christ came into the world through his reincarnation as human being via the virgin Mary. The purpose of his coming is to become a ransom for the sins of the people of the world because everyone is a sinner at birth as a result of being the descendants of Adam and Eve.
Jesus Christ died on the cross for our sins to become a sin offering for everyone, like how the Israelite priest must offer a sin offering of bull for his sin and two goats (one goat as scape goat and the other as sin offering for the people) during the Day of Atonement. Blood is the requirement of atonement to sin in the Old Testament so Jesus Christ fulfill this requirement by shedding his blood on the cross for our sins (Hebrews 9:7-14; Hebrews 9:22).