I enrolled in a course Elaine Pagels taught at Union Theological Seminary in New York during my last year there because I'd read some of her work on gnosticism as a part of preparing an undergraduate thesis on scholarly opinions regarding the origins of the phenomenon. Additionally, I'd come to know her a bit socially because my girlfriend had studied with her at Barnard College previously.
The course itself was ostensibly about Genesis, about materials which appeared in Pagel's Adam, Eve and the Serpent, but I enrolled only after receiving permission to concentrate instead on reading all the the ante-Nicene patristics in order to compile what I eventually entitled "On the Procession of the Heresiarchs of Gnosis"--an exhaustive compilation of all patristic texts pertaining to the subject.
Most of the early Church Fathers are very, very boring. Few wrote well. Excepting perhaps Origen, few even remotely approach the learning and intellectual acumen of a Plato or Aristotle. Consequently, the work was tedious, something I never could have done were it not for the project of seeking relevant texts, gems among the dross.
Most important because very early and quite extensive (2nd century) was Irenaeus, a bishop of Gaul. His Adversus haereses was filled with references to various "gnostic" teachers, both contemporary and historical. Like most writers of the period, Irenaeus saw belief systems, philosophies and ideologies as passing from teacher to pupil. In the case of "gnosticism", the origin, because biblical, was Simon Magus (Acts). Satanically "puffed up" with false pride, intent to feed on the body of the Church, his teachings spread like a cancer, metastasizing throughout the Christianizing world, proliferating to legions of sub-sects, each with its own leaders. Irenaeus' aim was to expose, ridicule and refute the teachings in order to maintain his own authority and that of his fellow bishops. This was, of course, more than a century before the Church became officially recognized in the Roman Empire.
Irenaeus is, as aforesaid, an important source, but he is not a good one. Not only does he come across as authoritarian and intellectually narrow, but his attempts to ridicule what and whom he believed to be heretical are themselves often ridiculous. Unfortunately, because most everything we knew about early "gnosticism" was, until Nag Hammadi, from patristics like him, his importance as a primary source led centuries of scholarship down wrong paths, one of them being the mistaken belief that there was something like a Gnostic religion which might be regarded as analogous to what became the Catholic.
Ironically, some of Irenaeus' own beliefs are themselves, albeit by standards established later, though still maintained, quite absolutely heretical. Most notably he appears to have taken it for granted that Jesus lived to a ripe old age--indeed, Jesus had to in order to serve as an archetype of humanity.
If you want some idea of how very different the Church of the second century was from that after Constantine, then you might want to read Irenaeus.