And here we see a good example of why certain writings were not included in the Bible. Not because of their content but because no one in the Christian Church at the time the Bible was codified took them seriously as inspired Scriptures. They were just fantastic forgeries, and were treated as such. You might as well ask why the New York Times doesn't cover all of those "important" stories in the tabloids: because the tabloids make up their stories just to sell papers!

And by the way, the gnostics are still among us. No heresy ever dies, it just changes its marketing strategy.