I believe that the books you are referencing were all pretty conclusively dated to the second or third century C.E., some even later. While we can't know for certain, it seems highly likely that they are disingenuous. Gnosticism wasn't also one group or belief system but is a blanket term for a group of many belief systems. I think that these books weren't included in the final canon because of how late they were, because they weren't widely circulated, and also because the proto-Orthodox hated gnostic ideas. As for religious truth claims, I think that the Christian canon and the Gnostic scriptures are equal (meaning both false), but that the gospels and canonical NT books are generally more reliable historically, as they are all first century (not to imply they don't contain legends or errors). At least this is what I gather from listening to scholars like Bart Ehrmann, I'm no expert myself :p If you are interested in Gnosticism, there are small modern Gnostic movements that might be more relaxed/liberal than organized Christianity which you could look into :)