Paul did not receive his gospel from the other apostles, but was given a unique message directly from the risen Lord Jesus Christ. This message, the revelation of the mystery, differs from the ministry and message that the twelve apostles preached to the Jews. After Paul's salvation, he immediately began to preach the gospel of the grace of God to the heathen. A message that would place them in liberty in Christ.
When Paul encountered false brethren who sought to bring believers into bondage by the law of Moses, he stood firmly for the truth of the gospel that He received from the Lord. Adding the works of the law to God's grace robs believers of the liberty and riches they have in Christ and corrupts their service to the Lord.