The faith is the basis of our salvation. Differences of scruples and convictions about the teachings were matters of faith. Romans 14 deals with this point. In a fuller definition, the gospel is the good news of the Sonship of Jesus, His atonement, His resurrection, His glorification, and His promise to raise the dead. One cannot deny any element of this and be saved for that would be a denial of the saving role of Jesus, not just a denial of facts.
Facts have no saving power.
The gospel was not preached to the church. There is no record of such, no instruction for it, and no need for it. Although the word preach is used over one hundred times in the New Testament writings, it is not used in reference to a believing assembly. It is the word for evangelize . Other verbs relate to the edification of the saints.