Matthew 4:23
And Jesus went about all Galilee,teaching in their synagogues,
and preaching the gospel of the kingdom,
and healing all manner of sickness and all manner of disease among the people.

As you can see,Jesus preached about the kingdom already.
He already preached the gospel of the kingdom.

Matthew 24:14
And this gospel of the kingdom shall be preached in all the world
for a witness unto all nations;
and then the end shall come.

Jesus is referring to preaching what he taught.
He is referring to teaching the full message.
To teach the gospel of the kingdom
one must teach the gospel about the kingdom.
It is so simple,yet the many need to hear it.

John the Baptist taught of the kingdom.
Jesus taught of it.
When will it happen again on this earth?
We just need to teach it before the end can happen.
When the end comes depends highly on when this gospel is preached.

In today’s world the gospel of the kingdom can be a witness.
It means be something to be noticed by others.