"On the contrary, we speak as men approved by God to be entrusted with the gospel. We are not trying to please men but God, who tests our hearts."- 1Th 2:4
The greek word for gospel here is εὐαγγέλιον or euangelion. And at this point in Paul's missionary journeys it has taken on a specific meaning that was sufficiently standardized so that Paul's readers knew exactly what he meant. The word has two basic senses: active proclamation of the message and the content proclaimed. You can see both senses being used in 1Co 9:14.
To many, The Gospel is thought to refer specifically to Jesus' death, burial and resurrection. But that is merely a part of The Gospel. To better understand why the Jesus and the disciples used the word euangelion, we should briefly look into how the word was used in Jesus' day.
The lexical root of εὐαγγέλιον signifies notions of glad tidings and joyous news that is declared to others. In the Greco-Roman world, the noun εὐαγγέλιον was primarily associated with positive news in general and news of military victory in particular.
Εὐαγγελίζομαι denotes the activity of the εὐαγγέλος, the messenger of ancient Greece who was sent from the field of battle by ship, by horse, or as a swift runner, to proclaim to the awaiting city the victory — εὐτυχής is commonly associated with the announcement — of the army or the death or capture of an enemy or some other significant announcement. The noun εὐαγγέλιον, an adjective used as a substantive, derives from εὐαγγέλος and means simply “that which is proper to the εὐαγγέλος”, thus allowing the two-fold usage of antiquity, “reward/offering for tidings” and the “tidings” themselves.
Importantly, when the good news is associated with news of military victory, it is sometimes invested with religious connotations like divine favor and a gift of providence.
Interesting. There's much more to be said about "The Gospel". What do you think about this? What kind of military victory do you think the Kingdom of God had? Was the significant announcement of The Gospel the impending death of Jesus? Tell us what you think!
Christian Theology 3rd Edition, Millard J. Erickson (p. 982)
Bird, Michael F.. The Gospel of the Lord: How the Early Church Wrote the Story of Jesus (p. 6). Wm. B. Eerdmans Publishing Co.. Kindle Edition.