The Apostle Paul summarized the Gospel that he preached in 1 Corinthians 15:1-8. Basically, that Christ died for sins (according to Scriptures) and that he was buried and rose again (also according to Scriptures) and he was officially seen by the witnesses listed (including by Paul himself). This is also basically the gist of the Apostolic Sermons (of Peter, Stephen and Paul) described in ACTS. In each of these we see the following with only slightly varying emphasis: Jesus of Nazareth being the fulfillment of the promises in the (OT) Scriptures, being crucified and resurrected and then ascending to the Father, and sending the promised Spirit (along with the commands to repent, believe and be baptized for the forgiveness of sins in response to this message) and that they were witnesses of these things. As CS Lewis put it in the SCREWTAPE LETTERS, the earliest Christian message was largely based on the one historical fact of the Resurrection of Christ, and on one major doctrine--the Atonement. If either of these are missing, then it ain't the Gospel.
Is there more to it? I would certainly think so. Since the message involves God, Jesus, fallen man/sin , the Scriptures, the Spirit, and salvation (as well as the expected response of faith/repentance and baptism), then certainly there should be an agreement about what all of these mean. Who is 'God'? Who is 'Jesus'? What is 'sin'? What's man's condition? Who is the Spirit? What is 'faith'? How is one 'saved'? Which 'Scriptures'? And since folks who were baptized were then said to be added to the Church (ACTS 2): what is 'the Church'? What is the 'breaking of bread'? What is the Apostles' Teaching? If groups have contradictory views on what one or more of these things mean, then it's hard to see how they are proclaiming the same 'Gospel'.
Some of the earliest heresies indeed began with folks giving profoundly different meanings to what the Apostles were proclaiming/teaching about the various aspects of the above. The Judiaizers thought that keeping the Mosaic Law was a requirement for salvation. The Gnostics in their various forms taught a 'Godhead' which differed significantly from the God of Abraham, Isaac, and Jacob, and a 'Jesus' with a different mission and origin that the Jesus of Nazareth proclaimed by the Apostles. Marcion explicitly taught that the God of the OT was NOT the same of the Father of the NT, and he abridged his Scriptures accordingly. The Ebionites denied the true Divinity of Christ; the Docetists, His true humanity; and so on down through time.
In our modern time the various alternatives have multiplied. So how does the seeker know which "Gospel" is the true one? "AD FONTES"---go back to Scriptures and the earliest Apostolic proclamation about Jesus, and see how they were consistently identified and interpreted by the Apostolic Churches in history from the first century in Jerusalem and going forward. (A lot more could be said, but alas, lunch break is over...)