I’m not sure how you conflate the people we call ‘Gnostics’ - about whose beliefs we really understand so little - with ‘postmodernism’.
Postmodernists don’t seem to have faith in much at all, but mere human intellect; descendants of the earliest Gnostics died for their Faith.
But, carry on.
The belief of the Gnostics is no mystery.
They believe in denying Christ. Nothing more to it.
Postmodernists are the same. It might seem to come from a different angle, but they seek the same end and like the Gnostics, with their methods they succeed only in shooting themselves in the foot. They don’t know it but they’re bleeding to death.