Acts 9:22, Acts 17:2-3, Acts 18:5, Gal 1:6-9
The gospel Paul preached is that the Christ prophesied in the Old Testament is Jesus. (Acts 9:22, Acts 17:2-3, Acts 18:5)
Acts 9:22 But Saul increased the more in strength, and confounded the Jews which dwelt at Damascus, proving that this is very Christ.
Acts 17:2 And Paul, as his manner was, went in unto them, and three sabbath days reasoned with them out of the scriptures, 3 Opening and alleging, that Christ must needs have suffered, and risen again from the dead; and that this Jesus, whom I preach unto you, is Christ.
Acts 18:5 And when Silas and Timotheus were come from Macedonia, Paul was pressed in the spirit, and testified to the Jews that Jesus was Christ.
However, there was a gospel that preached differently about Jesus. The other gospels will be cursed. (2 Cor 11:4, Gal 1:6-9)
2 Cor 11:4 For if he that cometh preacheth another Jesus, whom we have not preached, or if ye receive another spirit, which ye have not received, or another gospel, which ye have not accepted, ye might well bear with him.
Gal 1:6 I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel: 7 Which is not another; but there be some that trouble you, and would pervert the gospel of Christ. 8 But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed. 9 As we said before, so say I now again, if any man preach any other gospel unto you than that ye have received, let him be accursed.