During the Tribulation, the message what
will be preached to the world is called "The
Gospel of the Kingdom."

It is the same message that Jesus presented
to the nation of Israel in His earthly ministry.

Particularly as it is given in Matthew's Gospel.
The earthly ministry of Jesus was the King
speaking, "From that time Jesus began to
preach, and to say, Repent; for the kingdom
of heaven is at hand". Matthew 4:17