Next is chapter 6 of Scot McKnight's King Jesus Gospel
Intro: Evangelism Explosion
Chap. 1: The Big Question: What is the Gospel?
Chap. 2:Gospel Culture vs Salvation Culture
Chap. 3:From Salvation to Story
Chap. 4: The Gospel of Paul
Chap. 5: Salvation Takes Over the Gospel
Now "The Gospel in the Gospels."
In this chapter, McKnight argues that "the Four Gospels and the gospel are one" (81). These books are called "the Gospels" because they are the gospel (80). This, he argues, is why the early Christians called them each a Gospel because they are the gospel, the story of Jesus including his death, burial, and resurrection. This statement is striking: "they didn't call the first four books of the New Testament the 'Gospels.' Instead, they called each one of them the 'Gospel'" (81).
The Gospels are often said to be "a Passion story with an extended introduction" (83). So, "The Gospels are called 'The Gospel according to...' because they declare the Story of Jesus according to the apostolic script: the death, burial, and resurrection of Jesus--and this all according to the Scriptures" (85). "One could say the four Gospels are extensive commentaries on 1 Corinthians 15 or the apostolic gospel tradition" (90).
I've written a bit about how I see the use of the word gospel developing recently--here's a suggestive summary. I would have started with Jesus and watched the use of the term grow over the course of the New Testament. I see it having related but nevertheless developing nuances. So at some point it takes on the imperial connotations of Luke. It originally did have connotations of the restoration of Israel, I think.
I do think that Mark 1:1 is an important moment in the development of the word, where the good news about the arrival of Jesus, messiah gets attached to a book. I suspect that the use of it thereafter in reference to Matthew, Luke, and John followed naturally.