Is Paul's Gospel based on the Old Testament prophets or the Abrahamic Covenant?

Paul's Gospel is not based on the Old Testament prophets or the Abrahamic Covenant. According to Romans 16:25, it is the preaching of Jesus Christ according to the revelation of the mystery, something kept secret in God's mind since time began. Paul's message rests on that revealed mystery, not the prophets or the Abrahamic Covenant.

Les teaches that Romans 16:25 contains language few people read or comprehend. Paul calls it “my Gospel,” the preaching of Jesus Christ, and emphasizes that it is not according to the prophets, not the Old Testament, and not the Abrahamic Covenant. Instead, Paul’s Gospel is according to the revelation of the mystery, something that was kept secret in the mind of God since time began. The contrast is therefore between Paul’s revealed mystery and the earlier prophetic or Abrahamic material.

Romans 16:25

Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began,

Was Paul's gospel revealed through the Old Testament prophets?

No. Paul's gospel is the preaching of Jesus Christ according to the revelation of the mystery, which was kept secret since the world began. It is explicitly not according to the prophets, the Old Testament, or the Abrahamic Covenant, as stated in Romans 16:25.