Gnosticism is an important point to make in invalidating the new testament, though it does not do it alone. The entire idea that a few people in a random kingdom a few hundred years after all of these works were written for some reason was allowed to decide what is or isn't true in the Universe is insane.
Gnosticism is Post Christian belief that stems from the supposed writings of Thomas and to lesser extent the book of James. Gnosis, which is means knowledge in Greek, is the source of what Gnostics use to base their belief that one can "transend" this world of world of depravity and suffering and embrace the the spiritual. This includes practicing philanthropy to the point of personal poverty, sexual abstinence, and diligently searching for wisdom by helping others. To reach a level of godliness, the Gnostic must, self sufficiently, reach oneness with God through the "word" and stoicistic methods.
Their are no known pre-Christian gnostic texts found to date so Gnosticism cannot "invalidate" N.T.
Gnosticism does not invalidate the New Testament, but rather supplements it. Much of Gnosticism is based on the Gospel of Thomas, a book of sayings by Jesus. These provide an alternative view of Jesus's beliefs, that can be compared with the image of Jesus presented in the other four Gospels. While it demonstrates much of Jesus's philosophy, nothing in it contradicts his divinity.
No, I do not think that Gnosticism invalidates the New Testament, because the New Testament warns that some people will not believe. The New Testament also says that those who believe, even though they are not seen, are blessed. There will always be some who do not believe the message that is in front of them.
The Christian New Testament was never meant to be taken as a literal document, as scriptures of other religions were never meant to be taken that way either. It stems from an oral tradition based in mystical experience. The Gnostic gospels and doctrines just make that more clear but it scares the fundamentalists.