I've read some books on Gnosticism, including Hans Jonas's book, and I'm familiar with the view that it's a controversial category (there's a book that makes that case). If you look at the Gnostic Bible or some collection of the Gnostic texts, it looks like a grab bag, but the same can be said about the New Testament texts, and "New Testament studies" is certainly regarded as a valid discipline.
These semantic issues are largely dependent on people's interests. If enough people are interested in a subject, their dedication will make the subject legitimate. With the modern freedom of thought, there's plenty of interest in so-called Christian heresies.
I haven't written much about Gnosticism, but the subject comes up in one of my older articles that talks about the Troubadours.
