It's not TO the Gentiles, it's ABOUT the Jews. A book can be written about you but not to you!!
Paul was one of the very top Jewish theologians that God had drafted to take Gentile believers, and convert them to the ways that Jews had known for centuries. Paul being a Roman citizen was the perfect choice, having known both worlds.
On the other hand...
Jews were not needing as much what the Gentiles needed concerning how to live as people of God.
So, for the Jews?
God drafted Peter to go to the Jews with a simpler message on one hand, but also embedded with revolutionary Church age truths.
In the end, after time passed. God had both teachings merge into one system for all the Church.