In Context: 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.
Already, men were adding requirements to the Gospel preached by Paul and the rest of the Apostles. About 1800 years later, the Catholics added even more.
An example would something like adding titles/attributes to Mary OR forcing priests to be celibate as a requirement of servitude. There are plenty more that come to mind.
Preaching is oral. How do you know what Paul preached? Got transcripts?