The word euangelion/gospel (literally, good news) appears 76 times in the New Testament.
I think it's always interesting to examine the first appearances of key concepts in Scripture.
Here's the context of the first appearances of "gospel" in Matthew, Mark, Luke and John:
Matthew
4:23 And he went throughout all Galilee, teaching in their synagogues and proclaiming the gospel of the kingdom and healing every disease and every affliction among the people.
Mark
1:1 The beginning of the gospel of Jesus Christ, the Son of God.
1:14-15 Now after John was arrested, Jesus came into Galilee, proclaiming the gospel of God, and saying, “The time is fulfilled, and the kingdom of God is at hand; repent and believe in the gospel.”
Luke
The noun form of gospel doesn't occur, but the verb form does:
The first to Zechariah. 1:19 And the angel answered him, “I am Gabriel. I stand in the presence of God, and I was sent to speak to you and to bring you this good news.
The second to the shepherds in the fields. Luk 2:10 And the angel said to them, “Fear not, for behold, I bring you good news of great joy that will be for all the people.
John
None of the forms of the word "gospel" appear in John!
Interesting!
In Matthew, Mark, and Luke, the word "gospel" is most often attached to language about the "Kingdom of God" or the "Kingdom of Heaven". And it seems there are some historical connections between "good news" and human kings of the day. Worth exploring!
But all this raises the question in my mind: Why does modern talk of "gospel" sharing so often leave out any mention of the Kingdom?