Are they considered on topic? They are not primary texts as they do not appear in any Judeo-Christian canon. However, we also allow the direct examination of secondary texts. Should Gnostic texts be considered valid secondary texts that are open for direct examination?
ScottS wrote the following in a comment on another answer, which I think is very clear and helpful (emphasis mine):
...they are undoubtedly a special category of tertiary, as the controversy around them is whether the "Gnostic gospels" deserve a canonical (primary) position, and if secondary writings from those therefore deserve such status. So it seems a level of "direct examination" should be allowed in the context of discussing how/why they did not historically meet canonical status, while avoiding direct examination of text critical matters related to them
The whole area is rather specialist and not necessarily of direct interest to our target expertise, but like ScottS, I think these texts are a special category because of the questions on canonicity.
I propose that we go further though, and allow any question on Gnostic texts that intersects with our core subject, not just those that directly ask about how/why they did not historically meet canonical status. By this reasoning, we would also allow questions
- on comparative theology between the Gnostics and mainstream texts
- about authorship and date of authorship
- on the Gospel of Thomas (see David's comment below for why)
and the like.
This is similar to a loose interpretation of Dan's 'tertiary' answer minus the word 'indirect':
The Gnostic texts should be considered tertiary texts and are thus only open to
indirectexamination where they help shed light on primary or secondary texts.
I don't think the distinction between secondary and tertiary text has any particular bearing on this issue. What matters is whether the expertise necessary to construct informed answers on topics ranging through manuscript evidence, historical religious context, translation, and interpretation is the same expertise that we apply to texts belonging to various Biblical canons.
In keeping with this earlier guidance from SE staff, I think we should scope the site around the area of expertise rather than the covers of any particular book. Questions about about apocryphal texts such as the gospel-of-thomas or even the epistle-of-barnabas seem like they should be on topic for both direct inquiry as well as in relation to our primary sources.
While allowing for examination on Gnostic gospels, using areas of expertise to define the scope of the site does rule out the inclusion of most other religious texts on the basis that the languages and religious context are so different that a different area of expertise is necessary to handle them.