The Mandaeans of Iraq can be described as a living Gnostic religion that preserves some very ancient Gnostic scriptures. One of those is online at the Internet Archive (Archive.org), and is in English, The Canonical Prayerbook of the Mandaeans, by E. S. Drower.

But the collection that has received the most attention is, The Nag Hammadi Library, unearthed in Egypt back in 1945, around fifty books, many of them from two Gnostic sects: the Valentinians and the Sethians. Those are all online at Gnosis.org. They were placed into a clay storage jar and buried probably by monks in the fourth century. They most likely were books removed from the library of a nearby monastery about a mile and a half away from where the books were found. This monastery was founded by Saint Pachomius, someone claimed by Orthodoxy but who probably had been in charge of a community that’s not-so-orthodox by today’s standards of Orthodoxy given the interest of monks in these other scriptures and the contemplative spiritual practices they represented, sorta like monks reading the Tibetan Book of the Dead, making it a more eastern type religion. Late in the fourth century Christianity underwent some major changes, standardization and one-size-and-theology-fits-all conformity, and those books were no longer kept in the monastic library. The process of copying those kinds of writings seemed to mostly come to an end around that time. Most of the discoveries of Gnostic texts in Egypt represent Coptic and Greek writings that date back to the second, third and fourth centuries, and not much later.

The Cathars were one of the last surviving Gnostic movements, and existing much later than those represented in the Nag Hammadi, during the middle ages in France. One of their main texts is lost, but a couple of short ones are included in the book, The Gnostic Scriptures, Willis Barnstone and Marvin Meyer. That’s a great collection. I believe Gnosis.org also has some of that material from the Cathars.