Paul was a Pharisee. That meant he had more than a working knowledge of the Old Testament. He had achieved expert status. In Galatians 1:14, Paul doesn't say he was advancing in the knowledge of Scripture but advancing in the traditions of my fathers. Paul's opponents suggest that Paul's interpretation of the OT was wrong. Their solution was Jesus and circumcision. Paul's answer was Jesus paid it all.
The gospel is according to Scripture, not against Scripture. A proper interpretation of Scripture will lead to an understanding of the gospel of Jesus Christ, sent, slain, ascended, and soon coming.
He is the Word by which the Father brought all things into being.
He is the ark of safety that keeps us from the wrath of God.
He is the lamb provided for Abraham's son Isaac.
He is the lamb slain to save his people from the death angel.
He is the prophet like Moses come to fulfill the Law and Prophets.
He is the promised son of David building a temple of the Saints of God.
He is the lamb of God we behold to take away the sin of the world.
He said Moses wrote of me.
He said Abraham saw his day and rejoiced.
As Paul will say later in Galatians, He is the Israel of God.
As Sally Lloyd-Jones says in the Jesus Storybook Bible, every story whispers his name!
The gospel is not in competition with the Scriptures; the Gospel is according to Scripture! The gospel comes to us through revelation.
Galatians 1:17 says Paul went away to Arabia. What is in Arabia? Mt Sinai, the place the Ten Commandments were given. Could it be that Paul went and studied the Law at the place the Law was first given? After he met Jesus, after the Father was pleased to reveal his Son to Paul, as one untimely born, he went to study for three years! Where the disciples got every day with Jesus for three years, Paul took his knowledge of the Scripture and saw Jesus as their true fulfillment, and Paul studied and prayed and read the Old Testament as Christian Scripture! No wonder he went to the synagogues as he traveled as a missionary. He went to show them Christ according to the Scriptures!