Paul himself preached two different gospels. One was based on what OT prophecies revealed:
"Paul, a servant of Jesus Christ, called to be an apostle, separated unto the gospel of God, Which he had promised afore by his prophets in the holy scriptures, concerning his Son Jesus Christ our Lord, which was made of the seed of David according to the flesh; And declared to be the Son of God with power, according to the spirit of holiness, by the resurrection from the dead" (Ro.1:1-4).
When Paul preached in the synagogues he preached the following:
"And straightway he preached Christ in the synagogues, that he is the Son of God.....proving that this is the very Christ" (Acts 9:20,22).
On the other hand, when Paul preached to the Gentiles the gospel he preached is described as being a "mystery" or something which had not been revealed in the OT:
"Now to him who is able to establish you in accordance with my gospel, the message I proclaim about Jesus Christ, in keeping with the revelation of the mystery hidden for long ages past, but now revealed and made known through the prophetic writings by the command of the eternal God, so that all the Gentiles might come to the obedience that comes from faith" (Ro.16:25-26).
Here Paul speaks about that gospel:
"But we speak the wisdom of God in a mystery, even the hidden wisdom, which God ordained before the world unto our glory" (1 Cor.2:7).
So the gospel which Paul preached in the synagogues was a gospel which had been revealed in the OT and the one which he preached to the Getiles had been kept secret.