"It's the gospel truth".
To mention the Gospel is to call together in a word everything that a Christian's life and faith depends upon. When someone associates the doctrines they teach with the truth of the Gospel, they are ascribing to them the highest place of significance and worth and placing the strictest demands upon their hearers for accepting them as the truth - accepting them as much as the message of Christ Himself and of salvation.
Gospel means good news, or good message. Paul, Christ's choosen Apostle to spread His message among the non-jews, defined the message in simple and definite terms as being the death, the burial, and the ressurection of Christ(1 Corinthians 15: 1-8). That is the Gospel that He said he preached. Good news, he said, because by believing in it, we are saved. He may have taught many things subsequent to that message, but, so far as I know, he never called any of them the good news of the Gospel itself.
But that is what many people do in spreading their teachings about the Bible and what they believe. They so associate their teachings with the Gospel, so relate them to its ability to save you, say the Gospel is so dependent upon them, that to reject their teaching(they will tell you) is to reject the Gospel itself. Their teachings about the Gospel and the Bible are called the Gospel itself.
Paul said to the Corinthains that they stood, by believing in the death, the burial, and the resurrection of Jesus Christ.
We ought to be cautious of those who take the widest latitude in demanding of others that they accept their teachings about the Bible as 'the gospel truth' and intimate or declare plainly, that to reject them is to reject the Gospel as well.
Some people will stop at nothing to pursuade you to believe what they say is the very truth of God Himself.
Christopher Joel Dandrow