From I Thessalonians 4.5: "the Gentiles which know not God."
The New Testament makes a three fold distinction among humanity: Jews, Gentiles, and the Church. (cf. I Corinthians 10.32, " Give none offence, neither to the Jews, nor to the Gentiles, nor to the church of God.")
A "gentile" in the NT is someone who does not know God. (Jews who rejected their Messiah do not know God -- cf. I John 2.23: " Whosoever denieth the Son, the same hath not the Father" -- but their rejection is a sad and special case: they were entrusted with the "oracles of God" -- Romans 3.2 -- and their not knowing God is a willful rejection).