Gospel means literally “good news”. Even in the Bible it is used to communicate various meanings. For example, Paul uses it in Galatians as the message to Abraham about having his children bless the world:
Gal 3:8 And the Scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel to Abraham beforehand, saying, “In you all the nations shall be blessed.”
Gospel is a generic term, although Paul crafted his own meanings into the word. In the Greek world, gospel was considered more of a divine proclamation. We have evidence that the word was used in conjuction with the birth of Augustus. From a translation of the Priene inscription:
Since the providence that has divinely ordered our existence has applied her energy and zeal and has brought to life the most perfect good in Augustus, whom she filled with virtues for the benefit of mankind, bestowing him upon us and our descendants as savior—he who put an end to war and will order peace, Caesar, who by his epiphany exceeded the hopes of those who prophesied good tidings
[gospel]. . . . and since the birthday of the god first brought to the world the good tidings [gospel] residing in him. . . . For that reason, with good fortune and safety, the Greeks of Asia have decided that the New Year in all the cities should begin on 23rd September, the birthday of Augustus.
NT Wright, writes about this:
…in the context into which Paul was speaking, “gospel” would mean the celebration of the accession, or birth, of a king or emperor. Though no doubt petty kingdoms might use the word for themselves, in Paul’s world the main “gospel” was the news of, or the celebration of, Caesar.
… Politically, it cannot but have been heard as a summons to allegiance to “another king”, which is of course precisely what Luke says Paul was accused of saying (Acts 17.7). Practically, this means that Paul, in announcing the gospel, was more like a royal herald than a religious preacher or theological teacher.
This would shed some light on the use of the word “gospel” in the NT.