The beginning of the gospel of Jesus Christ, the Son of God.
So begins the Gospel according to Mark. We might have different ideas about what the term “gospel” means, but for a first century reader the “good news” (from the Greek εὐαγγέλιον) would bring to mind the ideas of a ruler and/or a great victory. 
A popular example that captures these Roman ideas from the same time period (9 BC) is the Priene Inscription. It is an engraved stone created in the city of Priene in Asia Minor to celebrate the birth of Emperor Augustus. It is variously translated, but the Greek word for gospel appears twice (noted in bold).
It seemed good to the Greeks of Asia, in the opinion of the high priest Apollonius of Menophilus Azanitus: ‘Since Providence, which has ordered all things and is deeply interested in our life, has set in most perfect order by giving us Augustus, whom she filled with virtue that he might benefit humankind, sending him as a savior, both for us and for our descendants, that he might end war and arrange all things, and since he, Caesar, by his appearance (excelled even our anticipations [of good news]), surpassing all previous benefactors, and not even leaving to posterity any hope of surpassing what he has done, and since the birthday of the god Augustus was the beginning of the good tidings for the world that came by reason of him …
Another contemporary example of εὐαγγέλιον being used in reference to a military victory is Plutarch’s Agesilaus chapter 33.4, describing a poorly rewarded herald.
Nay, even after the battle at Mantinea, which Thucydides has described, the one who first announced the victory had no other reward for his glad tidings than a piece of meat sent by the magistrates from the public mess.
If you are interested in more uses of εὐαγγέλιον outside the Scriptures, this blog post is an excellent resource that pointed me to the latter example and has several more.
When the first century reader hears the term “gospel” they would be inclined to ask who is this good news about? what king or ruler are we celebrating? That would be followed by the question – what great victory has been achieved and by whom? The gospel was not about something but about someone. Scot McKnight, in the King Jesus Gospel says
“To gospel” is to herald, to proclaim, and to declare something about something. … the gospel is to declare something about a Person. 
Mark’s gospel addresses these questions. Right away we are told that this good news is about Jesus the Messiah/King. Like a runner carrying news of a great victory from a battlefield, John the Baptist is the herald proclaiming the good news that the Messiah and Davidic King promised to Israel (and the world) is coming (Mark 1:2-6; Luke 3:18).
[gospeling is] narrating the saving, forgiving story of Jesus as the completion of Israel’s story.
The gospel is about the Messiah/King Jesus. And Jesus himself has come to announce good news about the kingdom of God. It is at hand. This comes out a few verses into chapter 1, which is the next time εὐαγγέλιον occurs in Mark (1:14-15).
Now after John was imprisoned, Jesus went into Galilee and proclaimed the gospel of God. He said, “The time is fulfilled and the kingdom of God is near. Repent and believe the gospel!”
The word εὐαγγέλιον will not appear again until verse 8:35 after the climatic confession by Peter that Jesus is the Messiah (8:29).
For the Jewish reader and those who experienced the events recorded in the Gospel of Mark, the expectations and ideals of the kingdom would have been something they grew up with based on the readings of the OT narratives, prophecies, and promises.
“Behold, the days are coming, declares the Lord, when I will raise up for David a righteous Branch, and he shall reign as king and deal wisely, and shall execute justice and righteousness in the land. In his days Judah will be saved, and Israel will dwell securely. And this is the name by which he will be called: ‘The Lord is our righteousness.’ (Jeremiah 23:5-6)
The Roman reader who is less familiar with the OT would be asking, if Jesus is the Messiah/King then what does He have authority over? Over the first few chapters of the book Mark lays out a narrative that answers that question.
- Jesus was anointed with the Holy Spirit as Messiah/King at His baptism (Mark 1:9-11; Acts 10:36-38; Isaiah 42:1-7).
- Jesus has authority over Doctrine (1:21-22, 27)
- Jesus has authority and victory over Demons (1:23-27, 34; 3:11)
- Jesus has authority and victory over Disease (1:31, 34, 40-45; 3:10)
- Jesus has authority and victory over Sin (2:5-12)
- Jesus has authority over the Sabbath (and Traditions) (2:23-28)
- Jesus has authority and victory over Satan/Strong Man (3:23-27)
- Jesus has authority and victory over Storms (4:35-41)
- Jesus has authority and victory over Death (5:35-43)
What is this? Who is this? This is no ordinary King. As we continue to study the Gospel of Mark we will continue to look at Jesus the Messiah who proclaims the good news that the kingdom of God is at hand.
 N. T. Wright, What St Paul really said, 1997, p.43.
 Ibid. 43
 Scot McKnight, The King Jesus Gospel, 2011, p. 49,93
 Ibid. 84