'Truth of the Gospel' in the Bible

but when I saw, they conformed not their conduct to the truth of the gospel, I said to Peter before them all: if you being a Jew, live after the manner of the Gentiles, and not according to that of the Jews, why do you constrain the Gentiles to judaize?

I have not written to you, as to people unacquainted with the truth of the gospel: but as to such who know it, and know that it cannot give rise to any imposture.