Proof of only one gospel message.
That the gospel of grace people talk about is the gospel of the Kingdom of God.
“But I [Paul] do not account my life of any value nor as precious to myself, if only I may finish my course and the ministry that I received from the Lord Jesus, to testify to the gospel of the grace of God. And now, behold, I know that none of you among whom I have gone about proclaiming the kingdom will see my face again” (Acts 20:24-25).
Jesus` death, burial and resurrection is apart of the Gospel message of the Kingdom of God. This is affirmed in the Prophets and Jesus` words himself.
44 He said to them, “These are My words which I spoke to you while I was still with you, that all things which are written about Me in the Law of Moses and the Prophets and the Psalms must be fulfilled.“ 45 Then He opened their minds to understand the Scriptures, 46 and He said to them, “Thus it is written, that the Christ would suffer and rise again from the dead the third day, 47 and that repentance for forgiveness of sins would be proclaimed in His name to all the nations, beginning from Jerusalem.“ (Luke 24 NASB). This is the same gospel preached in the beginning to Abraham.”And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, In thee shall all nations be blessed.” (Galatians 3:8)