Well, the proof was that it wasn’t a forgery. Whether or not what is written is to be believed is another matter. Not saying it is or isn’t true (what is written) but that is an important distinction.
Gal. 1:6-9
6 I marvel that you are turning away so soon from Him who called you in the grace of Christ, to a different gospel, 7 which is not another; but there are some who trouble you and want to pervert the gospel of Christ. 8 But even if we, or an angel from heaven, preach any other gospel to you than what we have preached to you, let him be accursed. 9 As we have said before, so now I say again, if anyone preaches any other gospel to you than what you have received, let him be accursed.
Gnosticism is not new. I has been around since the first century Church...!