The meaning of the word "gospel" has changed dramatically since Jesus's time. Today it means the Biblical books about the life of Jesus, and it is a metaphor for the "truth." Many also know that the word comes from the Greek word for "good news." But in Jesus's time, it referred specifically to the reward that messengers received for delivering good news.
The Greek word is euaggelion (εὐαγγέλιον), which primarily means a "reward of good tidings." This noun is used forty-times in the New Testament. The word occurs in in other Greek literature but strangely enough the references are mostly from Greeks and Romans, not Christians. This word was clearly not popular among early Christian theological writers. It wasn't until after the second century that this word was starting to be used to describe the four books of the Bible that described the life of Jesus. By then, Latin was becoming more common the Greek in religious writing.
The prefix, eu, means "good." The root word is aggelos, which means "messenger." It came to mean "semi-divine beings" that is "angels" in later use. However, in Jesus's time, aggelos did not mean "angels," though it is often translated that way. It only meant messenger. The ending of euaggelion, "ιον," is a diminutive that makes the word into a neutral thing. The literal sense is "little something for good messengers." The English word "gospel" comes from Old English. In Old English, the Greek word was translated as gōdspel (gōd "good" + spel "news"). Of course, by that time the word had lost its sense of being a reward.
Jesus's Use of the "Gospel"
The Greek word is only used eight times by Jesus. In his use, its use is connected both with "news" and "rewards."
Jesus uses the term to describe bringing a "message" in these verses:
Mark 13:10 And the gospel must first be published among all nations.
Mark 16:15 ... Go ye into all the world, and preach the gospel
Matthew 24:14 And this gospel of the kingdom shall be preached
Matthew 26:13 ...Wheresoever this gospel shall be preached in the whole world,
Mark 14:9...Wheresoever this gospel shall be preached throughout
He uses it to refer to "rewards" in these verses:
Mark 8:35 For whosoever will save his life shall lose it...
Mark 10:29 ...There is no man that hath left house or brother...
This verse connects it with "trust," which could refer either to the message or the reward:
Mark 1:15 The time is fulfilled, and the kingdom...
There is also a verb form of this word from the same root, euaggelizo. (εὐαγγελίσασθαί). It is used five times by Jesus. It is usually translated as "preach" or "preach the gospel." However, the ending of this word means "to be." So the literal sense of this word is "to be a good messenger." It has less of a connection to the idea of a reward. It is used in these verses:
Luke 4:18 The Spirit of the Lord is upon me,
Luke 4:43 I must preach the kingdom of God to other cities
Matthew 11:5 The blind receive their sight, and the lame walk
Luke 16:16 The law and the prophets were until John:
Luke 7:22 Go your way, and tell John
Notice that the first of these verses if a quote of the Old Testament, where the Greek word is first used in the Septuagint.
Communication in Christ's Era
In Christ's world, the "media" was the physical transportation of information and its communication in spoken words. While written messages were passed, this was not the normal means of communication.
Private message were often delivered verbally to the intended recipient. Public message were announced publicly in the town center. Written messages were carried, but most people could neither read nor write. They were recorded and read. There was no concept of "publishing" information. As we know it now.
The word translated as "the gospel" is not simply the Greek word meaning "good news," as it is usually represented. It is a longer word describing the reward one gets for bringing good news. It conflates the two ideas: that of good news and the reward the messenger gets.
It is interesting how the rewards for "news" has been reversed in modern society. Historically, good news was rewarding for the messengers. Bad news was not rewarded and the messengers could be punished. In today's society, the media is rewarded in ratings for delivering bad news. Hence it is in their best interest to find bad news and even manufacturing it.
So there is a sense in this statement that all different types of people and nations will incorporate Christ's message into their laws and as justifications for laws. We see this, for example, religious law of the Middle ages and in other forms of statism in more modern times. This philosophies all base their appeal on idea of universal charity and justice that was unknown before Jesus, but by making them compulsory, under the treat of coercion by the state, make them meaningless in terms of personal virtue and has been shown to be highly destructive in term of social progress.