'Truth of the Gospel' in the Bible

To whom we did not yield by submission, no, not an hour, that the truth of the gospel might continue with you.)

But when I saw, that they walked not uprightly, according to the truth of the gospel, I said to Cephas before them all, If thou being a Jew, livest after the manner of the Gentiles, and not as do the Jews, why compellest thou the Gentiles to judaize?