I will give my idea about the creation of the first gospel.
In the first century some opposition developed to the orthodox Jewish religious establishment and their stringent Torah laws. An opposition began in Samaria, possibly by Simon Magus of Gitta, and those opponents were called the Gnostics. They taught that the Jewish god of the old testament was an evil demiurge, and there was a more superior, unknown god who was a god of the "good". The superior god of the good sent his saviour son Chrestos (the good) to save the Jewish people from worshiping the old testament demiurge, and give the Jews the knowledge (gnosis) of the superior, unknown god. These beliefs were taught by Simon Magus (the "good" Samaritan) in Samaria and also in Rome when he went there.
The teachings were continued by Menander the Samaritan in Antioch and his student Saturninus, who then taught Gnostic beliefs to Basilides of Alexandria, who taught his student Velentinus, who later traveled to Rome at a time when Cerdo the Syrian and Marcion of Sinope were in Rome.
In the first century the Gnostic teachings started becoming popular in Antioch and also back in Palestine. After 70 CE with the fall of the temple and the loss of power by the Sanhedrin there were some Jews who tried to counter the Gnostic stories and defend the Jewish traditions, but also criticize the Sadducees and Pharisees for being too authoritarian and dogmatic.
Finally the oral stories were written down in a gospel, but which gospel and by whom? Since some of the stories in the gospel came from the works of Josephus, the gospel would have to have been written AFTER the works of Josephus were reproduced, UNLESS, UNLESS it was Josephus who wrote the gospel story that included events from his experiences in Galilee and throughout Palestine.
Which gospel would have been written by Josephus? Since Josephus did not believe in the messianic Jewish beliefs he would not have written a gospel about a Jewish messiah. Josephus never wrote about the biblical Jesus (despite the redactions added in the 4th century), but he did write about a Simon the magician who lived in the time of Felix the governor in the 50s, so he could have been familiar with the teachings of that Simon. But would he have written about a god of the "good" who was different from the old testament, Jewish warrior god?
When Jerusalem was surrounded and besieged by the Roman army in 70 CE, Josephus went around pleading with the Jewish people to surrender or else there was going to be a mass slaughter of the people in Jerusalem, but the Jews in Jerusalem did not listen to Josephus, and they suffered at the hands of the Romans. By this, Josephus demonstrated that he did not support the Jewish god of war of the old testament, which would indicate that he would have been open to an alternative god of good.
Writing a Gnostic gospel about a "good" god who sent his son as a saviour to try to get the Jewish people to turn away from their warlike ways and "turn the other cheek" would have been consistent with someone who saw repeated Jewish rebellions only leading to the slow elimination of the Jews. In opposition to the Gnostic pacifist message, some men created a story about a son of the Jewish god who was a Jewish saviour claiming he "came not to bring peace but instead a sword".
In the Gnostic gospel story there is a sacrificial execution demanded by the Jewish Sanhedrin in which the Jews are shown to be powerless in trying to kill an immortal son of the Gnostic good god, whereas in the Jewish version the Romans temporarily kill a messianic Jew, thus creating more enmity against the gentile Romans.
It seems to me the Gnostic gospel story existed many years before Marcion used it in his New Testament, along with ten letters of Paul. The Jewish biblical, gospel story was created as a Jewish version of the Gnostic saviour story, and it also used parts written by Josephus about his experiences in Palestine. Finally a merging of both the Gnostic and Jewish gospel stories created the biblical gospels used in the 4th century bibles, thus explaining why the gospel stories contain several contradictory messages.
OK, now you can start tearing my ideas apart.