There is significant scholarly debate around what Gnosticism is, and therefore what qualifies as a "Gnostic text."[1]
Gnostic texts preserved before 1945
Prior to the discovery at Nag Hammadi, only the following texts were available to students of Gnosticism. Reconstructions were attempted from the records of the heresiologists, but these were necessarily coloured by the motivation behind the source accounts.
The so-called "Codex XIII" is not a codex, but rather the text of Trimorphic Protennoia, written on "eight leaves removed from a thirteenth book in late antiquity and tucked inside the front cover of the sixth." (Robinson, NHLE, p.10) Only a few lines from the beginning of Origin of the World are discernible on the bottom of the eighth leaf.
Bruce Codex contains the first and second Books of Jeu and three fragments – an untitled text, an untitled hymn, and the text "On the Passage of the Soul Through the Archons of the Midst".
Oxyrhyncus 1: this is half a leaf of papyrus which contains fragments of logion 26 through 33.
Oxyrhyncus 654: this contains fragments of the beginning through logion 7, logion 24 and logion 36 on the flip side of a papyrus containing surveying data.
Oxyrhyncus 655: this contains fragments of logion 36 through logion 39 and is actually 8 fragments named a through h, whereof f and h have since been lost.