I would encourage you to read the Feser article I linked to above if you want a more precise definition of it.
There could disagreement on whether these sort of beliefs are true or heretical, but based on the details above I don't think the charge that it's simply an ill-defined boogeyman to point at in fear is true. There's very clear definitions being given here.
You'll notice that Feser himself does not consider them as such. He rightly calls them considered at a high level of abstraction that leaves out the many differences between the various specific Gnosticizing movements that have arisen over the centuries.
Even if, for the sake of argument, I conceded you that this list of characteristics might serve as some sort of "definition" of Gnosticism, I would still have every right to demand of you a PROOF that The Gnostic heresy is one that has recurred many times in the long history of the Church, under various guises – Marcionism, Manicheanism, Paulicianism, Albigensianism, Catharism, and so on.
There are several problems with this claim (as admitted by Feser himself when he says that "many differences are left out") ; as with many questionable claims, I have no objection to it per se as long as it's presented as a mere hypothesis and a possible explanation, but I am absolutely opposed to the usually dogmatic and all-embracing way it is presented by its proponents.