Gnosticism is a fascinating topic. It stands at the crossroads between Platonism, Christianity, Judaism, Classic and Egyptian mythology, an extraordinary puzzle that we will likely never be able to piece together.
Its inscrutable character is half of why I enjoy studying it. I am a programmer, not a philologist: my knowledge of Latin is embarrassing and my knowledge of Ancient Greek would likely be inadequate for participating in even the rowdiest of orgies. I know that I will never discover anything of substance in this field. But reading its history, it's quite obvious that every discovery settles one problem and raises ten others. Every scholar of Gnosticism is, in this regard, a beginner. Some just had some more time to dabble in it than I did. They are far more knowledgeable than I will ever be, but in the grander scheme of things I'm not much more uninformed than they are. Thus, I find it easy to be content with being a dilettante.
If you're curious about this topic but are put off by the crystal healing gang, and would like some real scholarly resources, here's what helped me.
Note that the recommendations below, especially the books, are relevant primarily as starting points, not definitive guides. For something that's been effectively dead for centuries, Gnostic religions are a remarkably active field of study, and our knowledge of the topic is constantly evolving. I've tried to err on the side of being conservatively outdated, rather than challengingly novel: as with all active fields of research, reading recent literature can be daunting, because it builds on a lot of contemporary context that's not easily accessible outside the world of academia.
First, you want to be familiar with major topics in the philosophy of Plato. This is a very deep rabbit hole: there are people who spend their entire academic careers studying that. It doesn't help that Plato's works are more than two thousand years old, and though astonishingly ever-relevant, they are also a product of their times. Plato's discourse is ironical and, at times, very hermetic. Reading his Dialogues, though definitely necessary (we'll come back to primary sources later), is rarely sufficient on its own. You'll want to read some commentaries as well.
Plato's works in general are relevant for the study of Gnosticism, but some are particularly relevant. Plato's cosmology was highly influential, which means you want to read Timaeus carefully. Francis Cornford's Plato's Cosmology: The Timaeus of Plato is a good study guide. It's a classic commentary which, while outdated in some regards, will give you a lot of good starting points.
Plato was influential not only in terms of ideas, but also in terms of how they are communicated. Luc Brisson's Plato the Myth Maker is particularly enlightening in this regard, and offers invaluable cues to deciphering the language of myth.
Also of possible relevance, but for different reasons, is Gregory Vlastos' Philosophy of Socrates: a Collection of Critical Essays, and some of the material in the Studies in Greek Philosophy, published after his death. Vlastos' ideas are controversial and not quite universally well-regarded. But his methodology, controversial though it may be, is a great exercise in discerning distinct lines of philosophical thought in Platonic texts. Studying Gnostic texts is a somewhat similar exercise at times, except you're trying to discern distinct lines of philosophical thought in Gnostic texts.
Later Platonic writers are also relevant. Dillon and Gerson's Neoplatonic Philosophy: Introductory Readings is a good, well, introductory reading, which covers Plotinus, Porphyry, Iamblichus, and Proclus. I like it because I think it does a very good job covering the historical background of (what we now call) neoplatonism, which is extremely relevant.
For a long time, the classical introduction to the study of Gnosticism was Elaine Pagels' The Gnostic Gospels. It's still a good book, and it's worth reading, if only because it will provide some useful context to her other books.
Birger A. Pearson's Ancient Gnosticism: Traditions and Literature is also a good choice -- in my opinion, a better choice today. It's a little hard to follow at times but, in its defense, it assumes very little prior knowledge, and that's a very ambitious goal for a book on this topic. It's also newer, and that may not seem like much, but the nag Hammadi library, which irrevocably shattered our understanding of Gnosticism, was only discovered in 1945. The nearly thirty years that separate the first edition of The Gnostic Gospels and the first edition of Pearson's book are practically half the time we've known anything substantial about Gnosticism at all. But novelty aside, Pearson's book is deliberately written to encourage you to read the primary sources, which is an extraordinarily valuable experience.
Pearson's book alludes to a bunch of interesting material that he explores in an earlier book, Gnosticism, Judaism, and Egyptian Christianity. This is not an introductory book, but if you read the other one, it's a good follow-up.
I've also heard good things about Kurt Rudolph's Gnosis: The Nature and History of Gnosticism. I haven't read it but I am familiar with Rudolph's work on Mandaeism, so I wouldn't be surprised if it were extremely good. It's an older book (contemporary with Elaine Pagels') but that shouldn't stop you.
If you're looking for an introduction that is at once lighter and broader, Karen L. King's What is Gnosticism? is also a good text. This is not so much an introductory text on Gnosticism as a brief historiography of the subject (albeit somewhat reductionist at times). This is more relevant than it sounds because Gnosticism is not just a very broad topic, but also one that hasn't been effectively practiced for centuries. So a lot of "knowledge about Gnosticism" is more aptly called "knowledge of what others said about Gnosticism over the years", and King's book is a good intro on that.
Read the primary sources.
I cannot say this often enough. Read them. Annotate them. Summarize them, cross-reference translations. Primary sources are our one uncontroversial source of knowledge and understanding.
Gnostic texts are difficult to navigate; many are incomplete, and they were passed down to us without much of the cultural context that made them make sense. Reading them is a puzzle, and owing to excesses of ancient writing styles and modern translations, it can be a frustrating puzzle at times. This makes would-be students avoid dwelling on primary sources too much early on, because it feels like you spend two days deciphering some obscure ten-page bullshit, when you could spend two days reading the distilled version and learn so much more.
Thing is, though, primary sources are our one direct line to the people for whom God was not dead and for whom what we call Gnosticism was not "a" -- "one of the many" -- religion but their religion and ideology. You will not be able to attend a Sethian ceremony and you won't be able to talk to a Sethian priest. Reading the same texts that they read is the closest you'll ever get to feeling what they felt and pondering what they pondered.
For Gnostic texts, The Coptic Gnostic Library is the most comprehensive reference. It's also expensive as fuck and hard to track down. If you don't have access to it in a library near you, and are not some magnate who studies the history of religions for fun, what you probably want is The Nag Hammadi Scriptures: The Revised and Updated Translation of Sacred Gnostic Texts Complete in One Volume, edited by Marvin Meyer. It has modern, excellent translations, and very helpful introductory notes.
You also want to have access to Plato's dialogues, and the works of later neo-Platonist philosophers. All have been published in countless editions. Some are better, some are worse, but they are all alike in one aspect: the translation of some term or another is either criticized by some, or lacking in nuance, so you will slowly find yourself just using the original Greek term for every concept. At that point, as long as you're in front of a computer, you will find yourself perusing the Perseus Digital Library more than your copy of the Dialogues.
You will also want a copy of the Bible. Any translation will do as long as it's reasonably easy for you to follow it. For older religious texts in the region that was that cradle of Gnostic faiths, Ancient Near Eastern Texts Relating to the Old Testament is a classic reference that should be easy to get a hold of.
Studying Gnostic traditions is extraordinarily rewarding. Some of the more interesting problems that scholars have pondered over the years include: