I'm trying to crank out something due a long time ago. For whatever reason, things flow out of me better when I at least think someone is listening. So here goes cranking:
The word gospel has meant different things in different contexts, most of which relate broadly to the idea of "good news." The gospels of Matthew and Mark both make the "gospel of the kingdom" the essence of Jesus' earthly teaching (e.g. Matt. 4:23; Mark 1:15). This fact suggests that Isaiah 52:7 provides key background to how Jesus saw his ministry. In particular, Jesus likely saw his ministry as part of the restoration of God's reign within Israel and the world, including the defeat of the evil powers that currently rule the earth.
The original context of Isaiah 52:7 was of course the return of Israel from captivity in Babylon around the year 538BC. The statement in that verse, "our God reigns," seems to connect directly to Jesus' preaching of the kingdom of God, the rule of God returning to earth as it is in heaven. Early Christians like Paul primarily read this verse in its Greek translation, where they found the verb "to proclaim the good news" (euangelizomai). It is thus quite possible that the early Christian use of the word gospel started among Greek-speaking Christians who connected Jesus' preaching of the rule of God to the Greek sense of a gospel as the announcement of good news of a momentous sort. For example, a famous inscription from 9BC about the Roman emperor Augustus of him as a savior whose enthronement was good news (euangelion) for humanity.
A major focus of the gospel in Matthew, Mark, and Luke is Jesus' ministry to the poor. The verb "to proclaim the good news" also appears in the Greek of Isaiah 61:1, which Luke pictures Jesus quoting as the very inaugural address of his ministry (Luke 4:18): "The Spirit of the Lord is upon me, because he has anointed me to bring good news to the poor." Matthew 4:13 also connects Jesus ministry of healing to his proclamation of the good news. Matthew, Mark, and Luke thus understood the gospel not only in terms of the coming reign of God over the earth but also in terms of the liberating ministry of Jesus to those who were oppressed and disempowered on earth.
In Jesus' ministry, the good news was about God's reign. When we get to Paul's writings, the gospel is about Jesus Christ himself as king. Romans 1:3-4 give us a good sense of what Paul means by the word. The gospel is about Jesus being king. He is descended from king David in his humanity and enthroned as "Son of God" as part of his resurrection. It is important to recognize that the title "Son of God" has everything to do with Jesus being king, being the Messiah, the Christ. The background for this phrase is firmly in Old Testament passages like 2 Samuel 7:14 and Psalm 2, both of which were originally about kings of Judah. Other New Testament books also picture Jesus' exaltation to God's right hand as a kind of enthronement as Son of God (e.g. Acts 13:33; Hebrews 1:5).
Paul's sense of the gospel of Jesus is thus much like the inscription about Augustus. The good news is that Jesus has been enthroned as king. As king he will bring justice and salvation to the earth. The gospel thus extends beyond the fact that Jesus is king to the benefits and blessings that have come in consequence.
Mark 1:1 probably has a similar sense of the word gospel as Paul. The beginning of Mark narrates the beginning of the good news that Jesus Christ is king. Perhaps it is then a coincidence of history that the word gospel came to refer not just to the good news of Jesus as king but to the genre of books like Mark that present the good news about Jesus. Thus we now refer to books like Matthew, Mark, Luke, and John as gospels, whereas originally their audience would have thought of them as biographies (Matthew, Mark, John) and a history (Luke-Acts).