What is the gospel of the grace of God in Acts 20:24?
It is the gospel of the uncircumcision that we find mentioned in Acts 15 and also in Gal 2:7. It is the good news that the Gentiles could become a part of the congregation of Israel and partake of their blessings (Romans 15:27) without being under the law. So in that early church the Jew walked by law, the Gentile by grace.