In a nutshell, here is the gist of my conclusions:
- A couple of uses of the word gospel in Greco-Roman culture are significant. There is of course the announcement of a victory in battle, a "gospel" from the war front, usually at the hands of a messenger. Even more significant is the Priene inscription, which speaks of the "gospels" relating to the birth of Augustus, the birth of a god and "gospels" concerning the good he would bring.
- The Septuagint uses the word gospels also in relation to relating victory in battle (2 Sam.), but the most significant uses are in the later part of Isaiah (e.g., 52:7; 61:1).
- Mark makes Jesus' good news proclamation of the kingdom the centerpiece of his message (1:14-15). Mark implies that this message significantly involves Jesus himself (e.g., 1:1), but we get the impression that Jesus himself focused much more on the kingdom as God's reign.
- Matthew does not add much to Mark, but includes proclamation of the good news of the kingdom of heaven as the centerpiece of Jesus' message.
- Luke highlights Isaiah 61:1 in its presentation of the inauguration of Jesus' ministry, making the good news about the reincorporation of the poor and displaced of Israel the centerpiece of the earthly good news.
- John does not use the word gospel, one of many instances of it following its own path. John is like the Message translation of Jesus--the most directly communicative about who Jesus is, but probably the least literal in its presentation of Jesus.
- If we then try to look behind the specific emphases of the gospels to what Jesus and John the Baptist might look like on video, I personally think that the language of gospel does indeed go back to Jesus' use of Isaiah 52:7 in his ministry. I believe with N. T. Wright that the context of John the Baptist is the restoration of Israel and its metaphorical return from exile. Paul and other Greek-speakers may have started using the noun "gospel," but I suspect their use derived from Jesus' use of the Aramaic of Isaiah 52:7.