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.

Les explains that Romans 16:25 uses language very few people comprehend or even read. Paul's gospel is the preaching of Jesus Christ according to the revelation of the mystery, not according to the prophets, the Old Testament, or the Abrahamic Covenant. That mystery was kept secret in the mind of God since time began. Les emphasizes that this is what the Bible says, not his own language.

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,

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.