The discussion of apocryphal texts is a complex one.
If you look at the teachings of the gnostics (ie the 2nd century sects, not the 'principle' of gnosis as such) they invariably try and 'hi-jack' Christian teachings and lead them off somewhere else.
Christ taught about love, and a one-on-one relationship with the Father - he told people to call God 'Abba'.
Gnostics teaching is utterly different - its all about 'knowledge', not love, so gnosis isn't who you are, but what you know ... in gnostic teachings there are inumerable intermediate levels that separate man from God ... syzergies, archons ... and to pass from one to the next you need 'keys' (knowledge again)...
Scripture, and Christ, taught that the world is essentially 'good' - the gnostic view is the world is essentially evil and the soul is trapped here ...
So 'basic' gnostic teaching is anti-Christian at almost every step.
A favourite technique of the gnostics was to write a 'gospel' in which Christ is utterly devisive, and imparts a 'secret teaching' to one, but not the others. So in one Judas is the 'specially chosen', in another it's Mary Magdalene, in another its Thomas ...
Another technique was to insist that although they never met Jesus, the gnostic teachers actually understood what Christ was teaching, whilst the poor old disciples were too stupid and hadn't got a clue ... that fact that 'their' Christianity bears a stunning resemblance to Zoroastrianism/Manicheanism and is nothing like the orthodox canon, nor the Old Testament, speaks for itself.
The most we know is from Irenaeus, who wrote a refutation of everything they taught. Irenaeus was a student of Polycarp, who was a student of St John, and it is likely that Irenaeus was in communication with people who had actually met Jesus personally, or who 'knew someone who knew him' (and I'll admit that allows for a lot of rumour and misinformation)
But Irenaeus argued that wherever Christianity was taught, the message was the same, in Rome, Alexandria, in Gaul (he was in Lyon) in Antioch ... and yet all those Christians had all got it wrong, and yet wherever gnostics taught, and a 'rule' of gnosticism is that each teacher should make the message his own, and thus no two gnostic teachers taught precisely the same thing, they were all right!
His last argument was that Christ could not have been much good if he hadn't managed to teach even one of his disciples the right message.
And remember that it is axiomatic to gnostic teachings that only a very, very few are saved, because the vast majority of humanity do not have a soul...
Lastly, if you don't believe the Christians, read up on the Greek Philosophers. The Stoics especially ripped into the gnostics at every opportunity for what they saw was very bad philosophy - a lack of reason, logic and intellectual rigour - whilst they acknowledged that they might not believe in the Christian God, Christians argued the case perfectly.
The point is that some gnostics were Christians who tried to express what they thought through what they already knew, others were not Christian at all, but simply wanted to get on the bandwagon (Christianity was spreading like wildfire).
Personally, I am Catholic, but a Christian Neoplatonist, for me Platonic philosophy is far more a 'real gnosis' than the oooh-er mystery stuff the gnostics churn out, but it's tough reading. Gnostics always aimed at popularity, rather than reality.
And if I were to bang my gong, I'd want to know if people want to know the 'real deal' on gnosis, why, Catholics especially, aren't they reading Athenasius, Basil, Clement, Dionysius the Areopagite, Ephrem the Syrian, Eriugena or Eckhart, Gregory of Nyssa, or Nazianzus, (I know, I skipped the 'F'), Hilary of Poitiers, Ignatius, John Damascene or John of the Cross ... Katherine of Siena (I cheated, should be a 'C') ... can't think of an 'L', hang on, Leo the Great! ... Maximus the Confessor (who re-ordered Platonism - my hero!) ...
Hey ... where'd you all go... ?
Thomas