People will often begin talking about a concept by giving a dictionary definition. I have always found these definitions marginally helpful at best. They are normally shallow and wooden. Language doesn't actually work based on definitions, it works based on how the terms are actually used. So I will not give a dictionary definition of "gospel," rather we will take a very brief inductive look at what it seems meant to communicate.
In the New Testament “gospel” translates the Greek word evaggelion. This word simply means “news that brings joy.” In the Roman world the gospel was the announcement of the ascension of the emperor that went out to all the provinces to assure the people that the universe was intact and that the wonders and joys of the Pax Romana would flourish ever more under the bountiful grace of the newly reconstituted heavenly realm of Rome. The people were to be overjoyed at the good news that the great savior has come.
Incidentally, this news was not on the whole joyfully accepted. Though there were benefits and advances that came from the Romans, the experience of most people was that Rome came promising peace but all they saw was a bloody sword.
So when someone in the Roman world claims to declare evaggelion they expect what they are saying to bring joy. They are indicating that what they are saying deals with things of universal import. They are declaring something that has been accomplished. Jesus and the New Testament authors believed that nothing short of this was going on.
But what was going on?