Mat 24:14 And this gospel of the kingdom shall be preached in all the world for a witness unto all nations; and then shall the end come.
We know the disciples when into all the known world of that time.
It was the "gospel of grace" that went to all the world and not the "gospel of the kingdom":
"The word of the truth of the gospel; Which is come unto you, as it is in all the world; and bringeth forth fruit, as it doth also in you, since the day ye heard of it, and knew the grace of God in truth" (Col 1:5,6).
The Apostle Paul was given a ministry to the Jews and one to the Gentiles and in the following verse the Lord speaks about those two ministries:
"But the Lord said to Ananias, 'Go! This man is my chosen instrument to proclaim my name to the Gentiles and their kings and to the people of Israel" (Acts 9:15).
In the following passage Paul speaks specifically about those two ministries:
"But none of these things move me, neither count I my life dear unto myself, so that I might finish my course with joy, and the ministry, which I have received of the Lord Jesus, to testify the gospel of the grace of God. And now, behold, I know that ye all, among whom I have gone preaching the kingdom of God, shall see my face no more" (Acts 20:24-25; KJV).
The preaching of the "kingdom of God" was the same exact "gospel" which the Twelve preached to the Jews in the following passage:
"Then he called his twelve disciples together, and gave them power and authority over all devils, and to cure diseases. And he sent them to preach the kingdom of God, and to heal the sick...And they departed, and went through the towns, preaching the gospel, and healing every where" (Lk.9:1-2,6; KJV).
The facts reveal that when they were preaching that gospel the Twelve were not even aware the the Lord Jesus was going to die. After being given that command and after preaching that gospel the transgiguration followed (Lk.9:29-36; Mk.9:2-13). Then after the Twelve preached the gospel of the kingdom and after the transfiguration we read the following exchange between the Lord Jesus and the Twelve:
"They left that place and passed through Galilee. Jesus did not want anyone to know where they were, because he was teaching his disciples. He said to them, 'The Son of Man is going to be delivered into the hands of men. They will kill him, and after three days he will rise.' But they did not understand what he meant and were afraid to ask him about it" (Mk.9:30-32).
The facts reveal that the Twelve did not even know He was going to die as late as shortly before the Cross:
"Jesus took the Twelve aside and told them, 'We are going up to Jerusalem, and everything that is written by the prophets about the Son of Man will be fulfilled. He will be delivered over to the Gentiles. They will mock him, insult him and spit on him; they will flog him and kill him. On the third day he will rise again.' The disciples did not understand any of this. Its meaning was hidden from them, and they did not know what he was talking about" (Lk.18:31-34).
These facts prove conclusively that the gospel which the Twelve were preaching at Luke 9:6 was not the same gospel which Paul referred to in the following way:
"For the message of the cross, is foolishness to those who are perishing, but to us who are being saved it is the power of God" (1 Cor.1:18.).
The "gospel of the grace of God" cannot be preached apart from the fact that believers are redeemed by the blood of the Lamb (1 Pet.18-19) and that is exactly the same "redemption" Paul speaks about when declaring the "gospel of the grace of God," that believers
"are justified freely by his grace through the redemption that came by Christ Jesus" (Ro.3:24).