I think it can be summed up in Luke 4:
16 He went to Nazareth, where he had been brought up, and on the Sabbath day he went into the synagogue, as was his custom. And he stood up to read. 17 The scroll of the prophet Isaiah was handed to him. Unrolling it, he found the place where it is written:
18 "The Spirit of the Lord is on me,
because he has anointed me
to preach good news to the poor.
He has sent me to proclaim freedom for the prisoners
and recovery of sight for the blind,
to release the oppressed,
19 to proclaim the year of the Lord's favor."
20 Then he rolled up the scroll, gave it back to the attendant and sat down. The eyes of everyone in the synagogue were fastened on him, 21 and he began by saying to them, "Today this scripture is fulfilled in your hearing."
"To proclaim the year of the Lord's favor" (grace).
So, I wouldn't want to place a verse here and there in their own vacuum and treat the Gospel of the Kingdom as some difficult to see term. Rather I would bring the words having to do with it together to see what it details.
I could add John 3 to it, or much of it. The definition of the Kingdom of God is rather simple, I think. It is a realm which is as of now for humans, invisible to fleshly eyes. It is here in the spiritual sense, and yet it is to be physical in a time in the not too distant future. Saying that it is here in the spiritual sense is not to be taken lightly. It's just as meaningful now as it will be when it manifests physically later.
We who believe are declared citizens of a heavenly realm. We are not counted as part of the world as Jesus referred to the world in John chapters 14-17. We are His sheep as referred to in John 9 and 10. We are responding to Jesus in the way he illustrated as the Father gives us the words to say and how to say it, just as Jesus said the Father treated Jesus. So we who are in Christ act in the reality which is greater than the reality of what men can see and understand. We are actually in the Kingdom of God though we cannot see it. The Holy Spirit brings to us the ability to understand this, even if it is incremental.