I have some sympathy with the gnostics but am not in complete agreement with their teachings.
I do believe that all true know edge arises from within. That no knowledge that is imparted can be true knowledge until we internalize it. I find dogma and hierarchy to be somewhat detrimental to the development of the soul.
On the other hand, I don't believe the material world was created by an evil demiurge and that souls are trapped in matter.
Gnostic scriptures are more symbolic and that is one reason for their exclusion. Many gnostic sects felt Jesus was a pure spirit and because of their position on the material world, they felt there was no way a pure spirit like Jesus could take on the mantle of a material body. Because of this belief, they did not believe that Jesus died on the cross and one of the gnostic gospels has Jesus with his disciples watching himself be crucified and laughing at the idea of death and suffering. Which, not taking the story literally, but as an illustration, I can get behind the idea of suffering and death as an illusion of the material world. Or, as my understanding increases, the idea that suffering comes from how we think or choose to view events. That the material world is the slave of thought. But I digress.
At any rate, there are a lot of reasons for the exclusion of gnostic texts but I always see the gospel of John and the letters of John as fairly gnostic.
But another reason for their exclusion is the early church was at war with the dualistic heresy. After wiping out early gnostic sects, they reappeared again as the Cathers in Occitania in the 12th and 13th century and the Bogomils in the East. In the West, the Albigensian Crusade wiped out the Cathers and gave Occitania to France. I believe the Western church applied some pressure on the Eastern church to wipe out the Bogomils but I think the Bogomils flourished for a while longer and died out due to Muslim influence. Who knows, in some forgotten corner of the East some Bogomil sect might still exist.
Anyway, one usually doesn't include the scripture of a mortal enemy in one's own cannon, and I believe that is the main reason for the exclusion of the Gnostic scriptures. Still, like all forbidden works, they hold a certain fascination. For a while I had the Nag Hammadi library and used to enjoy reading it but I lost it in a move many years ago. It's a shame really because I think I would appreciate it better today. OTOH, I am most interested in reading other type of materials today so perhaps it is for the best.