"Gnosticism" is an umbrella term. The people originally and historically referred to as "Gnostics" never called themselves that.

They were the followers of (various) Greek mystery religions and attempted to integrate the Jesus figure into their belief systems as the latest teacher in their tradition from Pythagoras to Orpheus, for example.

But, obviously the Christian Universalists (aka Catholics) had a problem with that and got violent.

Those collectively called "the Gnostics" are these people - those who subscribe to the core premises of the mystery religions. Today's Gnosticism (legitimate Gnosticism does not operate on Catholic premises) can be described as a syncretism of these Greek mystery religions.