Galatians 2:7: But contrariwise, when they saw that the gospel of the uncircumcision was committed unto me, as the gospel of the circumcision was unto Peter;
Galatians 2:8: For he that wrought effectually in Peter to the apostleship of the circumcision, the same was mighty in me (e.g., Paul) toward the Gentiles:
II Peter 3:15: And account that the longsuffering of our Lord is salvation; even as our beloved brother Paul also according to the wisdom given unto him hath written unto you;
II Peter 3:16: As also in all his epistles, speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, unto their own destruction.
You are correct that Paul was an Apostle to the Gentiles, more explicitly to Rome.
Most early Church members in the body were originally Jewish.