Gnosticism has a wide following in the United States. Few people have real knowledge of theology or traditional Christian teaching or the Bible. It is no longer taught in the schools, and not deeply in the schools that purport to be Christian. They go to college where they encounter a general atmosphere of skepticism, where the professors, for the most part are unbelievers. The entertainment media are filled with persons who are actively hostile to Christianity. The amazing thing is that so many Americans retain their faith despite all this. But confronted with such new "facts." most are defenseless.
Actually, to an untrained eye, a Gnostic text would seem perfectly 'orthodox.' What betrays Gnostic heresy in a text are specific words used in a specific context, which resemble the words used in orthodox Christianity (i.e. Sofia, Wisdom).
Also one would find theological constructs which actually 'make sense' (for example, the infusion of secret knowledge, or gnosis about God, such as +Paul's instant conversion on the road to Damascus) and could easily be construed as the workings of the 'indwelling Spirit.'
Gnosticism also posits that there is God in each and every one of us. Thus, +Paul's statement (Gal 1:16) that "God revealed His Son in me" and not "to" me is one of the many reasons Gnostics claim +Paul as one of their own. But such claims, of course, do not prove that +Paul was Gnostic.
They do show, however, how delicate the line of separation could be (which is why it took the Church Fathers three hundred years to sift through some 200 scrolls and determine with certainty which of them were orthodox and which were Gnostic fraud).
Most church-going people do not realize that there is a fine line between an 'indwelling Holy Spirit' and 'God in me' concept.
Books talking Gnostic trash are like drugs: if there are no buyers, they will go away. Thus we can't blame America's Gnostic leanings on what's in Barnes and Noble; people buy them because they don't know any better, and they don't know any better because the "me-me'me" mindset believes that each man knows what's best for him/her.
So, as long as you have buyers, you will have Gnostic trash on the shelves (the business doesn't care what they sell as long as it sells, and if they don't someone else will).
vices will exist and sell as long as there are customers.