I was taught to categorize any group as gnostic if they maintained that 'secret', esoteric knowledge outside of what was revealed would lead to salvation.

Regardless of their practices all gnostic groups presented a big problem for the early Church. One end of the gnostic spectrum incest, murder, cannibalism, bestiality, etc., was alleged, drawing the ire of the State and giving Christians a bad name. The more scholarly, philosophical bunch were quietly undermining the authority of the Church, adding to the problems the early Church fathers were having. I'm sure you can all figure out what happened next...

While groups like the Valintinians are gnostic poster children, there was definitely something else going on in the shadows that theologians and philosophers are loathe to talk about, but Classicists won't shut up about. It survives by mention in letters between Church fathers discussing the 'issues' and Roman officials gossiping. Most of the good evidence is held at the Sackler in Oxford under lock and key.