I was asked to answer the following:
The question: What is the gospel according to the Bible?
The questioner has further qualified this question: What is the precise message that best encompasses the gospel?
The most succinct answer given in the Bible that encapsulates everything to do with the gospel is given by the Apostle Paul as: “the power of God to salvation for everyone who believes” (Romans 1:16).
Firstly, the Apostle Paul states that this gospel (good news) of salvation is available for everyone who believes. There are no exceptions.
Secondly, and most critically, while having precisely stated what the gospel encapsulates (salvation for all who believe); we need to clarify the following:
1) The power of God
2) Why salvation is needed
3) What it means "to believe"
It is in the clarification of the power of God, of why salvation is needed, and what it means "to believe", that varying views emerge. However, the Apostle Paul provides us with some assistance by informing us at the very beginning of the same chapter quoted above (Romans 1) that the gospel was promised beforehand through the prophets in writing (therefore historically verifiable), and concerned the Son of God and the implications of His resurrection.
The Son of God is Jesus of Nazareth, who is a descendant of the Jewish King David. However, it is through His resurrection from the dead, which was accomplished by His Own Spirit of Holiness that has truly designated Jesus to be the Son of God, the Savior and Lord of Life Who has conquered death. The good news being that all who exercise obedience of faith—in the reason the Son of God has appeared as a human in accordance to the Scriptures and the fact that He has demonstrated His power to overcome death—have a better hope other than the futility of being born to merely die like any other biological creature (Romans 1:1-5).
In the light of the above, to believe that salvation comes through the power of God is the gospel message and we can conclude without extensive elaboration that:
1) “To believe” literally means “to accept that something is true” and therefore comes with the conviction that there is no other option.
2) The reason we need salvation is that we are all born into a seemingly meaningless world of suffering that finishes in the grave.
3) The power of God is what is required to overcome death and provide life with meaning and this should provide an assurance that this hope is real and not vain philosophy.
Possibly the reason why there appears to be so many different versions of the gospel is that those who say they believe are not so keen on being transformed or, perhaps, do not understand that initially possessing the power to be transformed is not an actual fait accompli, but the prerequisite for the transformational process to begin.