Who is the Apostle of Gentiles?

In his epistle (letter) to the Romans, Paul refers to himself as an "apostle to the Gentiles."

Romans 11:13, 14 - For I speak to you Gentiles; inasmuch as I am an apostle to the Gentiles, I magnify my ministry, if by any means I may provoke to jealousy those who are my flesh and save some of them. [NKJV]