Paul's words to the elders of the Church of Ephesus contain an excellent principle for preaching the Gospel. "I did not shrink from declaring to you anything that was profitable and teaching you in public and from house to house, testifying both to Jews and Greeks of repentance toward God and faith in our Lord Jesus Christ" (Acts 20:20-21).
It has been said that the Gospel is Good News and not merely "good advice". The whole of the New Testament does give us a lot of good advice or spiritual guidance, but that is because the Good News is what it is, for our good.
The difference between the "Good news" and good advice is that the "Good News" of the Gospels is centred primarily on what God has done for us in Christ Jesus our Lord.
In contrast, large sections of the Epistles/Letters teach us to appreciate and respond to what God has done for us and continues to do unceasingly in the Church through the Holy Spirit, whom Jesus has sent to us from the Father.
Holy Spirit, enlighten us in our endeavours to live the Good News of the Gospel for our good.