You're absolutely right about the N O I. It's done a tremendous amount of good in individual lives. I didn't want it confused with international Islam is all I meant by my remark.
Jazz, Gnosticism wasn't a "Religion with a capital R" as Islam or Christianity, etc. It had many shapes and forms, perhaps the one unifying thing that it had that was shared more freely than other facets was its cosmology, but even that varied. There wasn't a "Gnostic Religion" that stood up and announced itself in that way. It was a religion of individuals far more than a religion of organization. It didn't form heirarchies of priests and cardinals and deacons and lay persons and sextons. It was more underground than overground. It is not easy to pigeonhole. It was indeed present before Christianity, or at least I believe that from my studies. A gnostic element could be present as an element in a lot of other systems.
I'd encourage you if you are interested to do some reading. The best books are still the ones by Jonas and Rudolph I would say, and the Nag Hammadi Reader edited by Robinson.