Did Jesus Teach Only Old Testament Doctrine?

There are many within churches of Christ today who teach that Jesus taught only doctrines relating to the old covenant, the Law of Moses. They will assert that since the Kingdom of God was not established until Acts 2, and therefore the new covenant begins in Acts 2, that anything and everything said or taught before this moment in Scripture ought to be relegated to the Old Testament and be regarded as Old Testament doctrine. The motive for this teaching invariably rests in Jesus' clear instructions regarding marriage, divorce, and remarriage, and this doctrine is an attempt to invalidate Jesus' teachings and to cause disharmony between the doctrines of Jesus and Paul. Is this what the Bible teaches? Let us examine the Scriptures and see what things are so.

We see that even from the beginning of His ministry Jesus points to the doctrines of the Kingdom of God. We read the following in Matthew 4:17, 23:

From that time began Jesus to preach, and to say, "Repent ye; for the kingdom of heaven is at hand..."
And Jesus went about in all Galilee, teaching in their synagogues, and preaching the gospel of the kingdom, and healing all manner of disease and all manner of sickness among the people.

We see, therefore, that from the beginning Jesus preaches the "Gospel of the Kingdom." Is this not the Gospel seen throughout the rest of the New Testament? Or does Paul condemn Jesus in Galatians 1:6-9?

I marvel that ye are so quickly removing from him that called you in the grace of Christ unto a different gospel; which is not another gospel only there are some that trouble you, and would pervert the gospel of Christ. But though we, or an angel from heaven, should preach unto you any gospel other than that which we preached unto you, let him be anathema. As we have said before, so say I now again, if any man preacheth unto you any gospel other than that which ye received, let him be anathema.