Why did Paul write to the gentiles?

God's salvation is available to all people Jew and Gentile. Paul was chosen by God to preach the gospel to the gentiles

Galatians 1:15,16 But when it pleased God, who separated me from my mother's womb and called me through His grace, to reveal His Son in me, that I might preach Him among the Gentiles, I did not immediately confer with flesh and blood,