The old 'Kill them all, God will know his own' approach by the authorities seems wasteful. But it does ensure nothing will be available for further examination.
So I'm going to pass on any discussion of Gnostic's, at this time.
Quite apart from whether or not you agree that the revolutionary and pernicious Manicheans should have been dealt with, and apart from whether or not someone actually said the words of that oft repeated quote, Gnosticism is a heresy and its doctrines would be odious even to the most Baptist of the Baptists some of whom claim their spiritual descent from these heretics.
Actually, there is something to that. Anabaptists were often just as violent and destructive as the Manicheans.