That’s because there are NOT two different gospels. The difference is how God deals with the people in each dispensation. Even the apostles wouldn’t have known what the “mystery” was until after the indwelling of the Holy Spirit. Prior to Pentecost they were still understood only the external laws.
Ephesians 3:2 If ye have heard of the dispensation of the grace of God which is given me to you-ward:
Paul preached the gospel/good news of grace which prior had been a mystery.
Acts 20:24 But none of these things move me, neither count I my life dear unto myself, so that I might finish my course with joy, and the ministry, which I have received of the Lord Jesus, to testify the gospel of the grace of God.
What does Paul mean by this? 1. The GRACE OF GOD WAS GIVEN TO HIM, BY GOD. 2. He (Paul) has laid the foundation of THE GRACE OF GOD. 3. But take heed how another builds on this foundation. 4. The foundation is Jesus Christ.
IOW: Jesus Christ is the foundation of the grace of God. Just as Jesus Christ is the foundation of the Law and the Prophets. The foundation of the grace of God is what the Church the Body of Christ IS. The foundation of the Law and the Prophets is what the Kingdom IS. Peter and the 11 built on this foundation as they preached the Kingdom message (The Great Commission). BOTH are built on the foundation of Jesus Christ. BUT they are DIFFERENT. One is for a KINGDOM of BELIEVERS. The other is for a BODY OF BELIEVERS. Which is what Jesus gave Paul, through direct revelations to give to the world...GRACE...the gospel of the GRACE OF GOD. Paul did not say that the foundation he laid was given to Peter, the 11, and Paul. He does NOT say that God gave this Grace of God to Peter, the 11, and himself. He says they are built on the same foundation, that being, Jesus Christ. This is why Matthew, Mark, Luke and John seem to never quite fit perfectly with Rom. through Philemon(water baptism for the remission of sins, "I am NOT sent but to the lost sheep of Israel", the day of Pentecost, etc.). The foundation is the same (Jesus Christ and His two-fold ministry), but the message is to different people, with different good news. A kingdom of believers and a body of believers.
The Gospel of Grace is the Finished Work of Jesus Christ. Grace “in action” was the Work of Christ on the Cross and Father raising Him from the dead three days later. The very sacrifice is the Grace we have. Peter and others also preached the same Gospel. The Acts are witness to people responding to Grace.