The Bible has a total of 66 books (39 in the Old Testament and 27 in the New Testament), authored by different individuals at different times. The New Testament itself has Gospels, Acts, Epistles, and Revelation.
When more broadly defined, the book of Acts belongs to Gospels, while Revelation, the last book in the Bible, belongs to Epistles because it is a letter written by apostle John on behalf of the Lord to His church. In this sense, the New Testament is basically consisted of Gospels and Epistles.
There is infinite divine wisdom behind this seemingly casual division. The Gospels recorded the historical events surrounding Jesus Christ when He lived on earth, and the historical events surrounding the Church of Jesus Christ during a period of time after Jesus was resurrected and ascended into heaven. These are the actual work done by the Lord Himself. While the Episodes explain the meaning of the facts.
When we study Gospel books, we notice that the focus of the Holy Spirit in the gospels is on what the Lord has done (which includes what He has said), not on explaining the meaning of what the Lord has done.
In a strict sense, Jesus didn’t mainly come to preach the gospel (the good news of salvation); He primarily come to accomplish the gospel, most specifically accomplish the victory that is the substance of the gospel. Without the actual victory, there would be no gospel, or at most there would only be a gospel in theory. This is analogous to a war situation in which the news of victory and delivered by a messenger, but the actual victory itself must be won in the battlefield.
What a victory has Jesus won! It has immense significance. In order for us to fully understand the victory and what it brought to us, God used apostles such as Paul to reveal the heavenly mysteries to us.
But we need to always bear in mind that the Apostles only explained what the Lord has accomplished. The salvation itself was accomplished (not merely preached) by Jesus and Jesus alone. Very often, people have a misunderstanding of this matter. They think that Jesus is like a great teacher who came to teach great doctrines, and his disciples further helped him to explain what he had tought. This would be a misunderstanding that is far from the truth.