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.
Galatians 1:6-9, 11-12
6 I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel:
7 Which is not another; but there be some that trouble you, and would pervert the gospel of Christ.
8 But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed.
9 As we said before, so say I now again, If any [man] preach any other gospel unto you than that ye have received, let him be accursed. ...
11 But I certify you, brethren, that the gospel which was preached of me is not after man.
12 For I neither received it of man, neither was I taught [it], but by the revelation of Jesus Christ.
Paul was well aware of the preaching of Jesus and of His disciples. It is that Gospel that motivated him to persecute and kill those who were followers of that preaching. We see here, then, that Paul obviously received something different than what was being preached by the eleven because he clearly states that what he preached he had received from no man, but only from Christ Himself. Those who followed the preaching of the eleven were also zealous of the Law of Moses:
Acts 21:20 And when they heard [it], they glorified the Lord, and said unto him, Thou seest, brother, how many thousands of Jews there are which believe; and they are all zealous of the law:
So, yes, the elements of works in the Kingdom Gospel, versus the singular element of faith in the finished work of Christ on the cross and in His resurrection, the differences become clear, which validate the words that Paul spoke to the Galatians as recorded above. Reiterating: Paul preached NOT what men had revealed to him, for he already knew what they believed. He preached the mystery that had not been revealed until then because of it being hidden in God. The exclusivity in what was revealed to Paul that was known to no man until then, the idea of it being the same as the Kingdom Gospel, THAT is what is moot as an argument in the face of overwhelming evidence to the contrary.
MM