Paul cannot preach a different Gospel, because he is not persuading man, but God (vs. 10). He is not out to please men, but God. If he pleases men by catering to them, taking their advice, or depending upon them for his spiritual understanding, he cannot be a servant of Christ. If he were a merely religious man, as he was in the past, then he could and would receive instruction from religious men with influence; but since he is preaching the true Gospel of Jesus Christ, then he cannot lower himself to the realm of man for the final proof of the evidence of the saving grace of God as it is related in the preaching of the true Gospel of Christ.
The Gospel that was preached to Paul was not from men (vs. 11). Man's gospel is one of socialism, a Social Gospel, which is so rampant in our own day. Jesus Christ has been removed from the central theme of most gospels today. In the minds of men, Jesus is no longer the Head of the Church, the only Savior of men, the way, the truth, and the life, and men have done as Israel, as Paul describes them in Romans 10:2, "and they, being ignorant of God's righteousness (Jesus), and going about to establish their own righteousness (not Jesus), have not submitted themselves to the righteousness of God (Jesus).
Man's gospel is no good for spiritual things: for salvation, for security, and for the deliverance of God into glory. Man's works fall short of God's righteousness; therefore, they are insufficient for their salvation, and their message is weak, pale, and ineffective to bring a man to God. Jonah explained it when he said in Jonah 2:9 "Salvation is of the Lord."
Neither did Paul receive this Gospel from man or was taught it by man, but he received it directly by a revelation of Jesus Christ Himself. We know the story of his falling to the ground as a result of the great light that shone from Heaven, Jesus being the revelation in the light by a voice that spoke to Saul, revealing himself to Saul as "Jesus, whom thou persecutest."
Paul was swept from his feet, struck with a great degree of weakness and dependence at that very moment, knowing that he must obey the heavenly vision; so he said, "Who art thou, Lord?" Jesus then announced his name "Jesus." Without Jesus, Paul could not rise from the ground, speak his own mind, or resist the vision. He was completely helpless to his own devices. He there surrendered wholly and completely to this Lord named Jesus.
Jesus, then, revealed the true Gospel to him just as he does to everyone whom he calls, and we were all helpless in that moment when the Spirit of God convicted us and wrought that great grace in our souls, convincing us of sin, and delivering us from the power of darkness, being brought into the marvelous light of the Kingdom of God. This is not of man but wholly of God. Thus Paul reiterates here in Galatians.