You chose...poorly.

The Gospel of Judas is certainly a later Gnostic work, but the Gospel of Thomas is one of the more compelling non-canonical works from the early Christian period. Even those who think it was written later than the canonical Gospels do not place it after 150 AD. Furthermore, although there are hints of Gnosticism in it, it is not properly a Gnostic work per se.

Furthermore, your argument against Gnosticism is a specious one that defies all of the rules of logic. I am not defending it, but your indictment of it is based on a reasoning that is beyond any attempt to make any sense of. Essentially, you say that the first reference to Gnostics in Paulinian writings is criticism. That would be like saying, in a world where Mormonism became dominant, "We know that Catholicism is wrong because the first reference to it in Mormon writings is to denounce it."

The reasons why Gnosticism is an unattractive candidate for metaphysical truth lies in its own internal contradictions.