Worth mentioning that Tom Schreiner published his NT Theology "The King in All His Beauty" back in 2014.

D.A. Carson has an article from 1996 on "What is the Gospel?" http://s3.amazonaws.com/tgc-documents/carson/1996_biblical_gospel.pdf

Does a study the word in the gospels, Paul, etc. plus conceptual analysis & distills:

Read the article and see how very present and central the kingdom of God and Jesus' kingship is.
ICYMI, Carson founded TGC.
So, at least one or two people in that orbit knew to talk about the gospel of the kingdom a while back.

It is the gospel of the Kingdom and the gospel of God's Son.

Not to put too fine a point on it, but Carson published this 15 years before McKnight's "King Jesus Gospel."