There are several things I'd like to note about the inaccuracy of this position for those of us invested in the historical hermeneutic:
1. Not every piece of literature in the NH collection is "Gnostic." The Thomasine literature is simply early Christian literature that represents the earliest form of orthodoxy in eastern Syrian around Edessa - a mystical form of Christianity that required celibacy to be admitted to the church. There is some Hermetic literature in NH collection (i.e., Discourse on 8th and 9th; Ascelpius). There is some Platonic literature (i.e., Republic). There is some early Christian (i.e., Teaching of Silvanus; Letter of Peter to Philip). To lump them all together as "Gnostic" and then ignore them is a way of marginalizing forms of Christianity that aren't familiar to us.
2. The word Gnosticism is a term relatively modern (18thc.) and it does not reflect the historical reality of the second century. There was no Gnostic religion separate from Judaism and Christianity or trying to pervert Christianity. Gnostic thought developed first within Judaism as a way to read the Bible literally while also maintaining the cosmology and anthropology of Middle Platonism. By the early second century, Christian theologians like Basilides and Valentinus who were philosophers were reading Christian scripture through this same lens. Orthodoxy did not yet exist, so there was no "real" Christianity to "pervert." There were many varieties of Christianity competing to control the Christian landscape. The "real" story is that Christianity was diverse in its early expression, and became more and more singular as borders were drawn and ideas and practices limited by very powerful bishops in the big metropolitan areas like Alexandria, Rome, and Antioch. This story is quite different from the one that these bishops claimed and which scholars for centuries bought into: that Christianity was singular in its early formation and that heretical thinkers (the Gnostics!) emerged along the way to lead good Christians astray.
3. The notion that Gnostic thinkers were perverting authentic Christianity is a theological position, not a historical one. It is a position with theological investment, that is, securing and maintaining the "real" and "biblical" Christianity of today.
4. If we want to know how the heck early Christianity formed into the type of Christianity it did, the second century is what we have to study. It is in the second century that the boundaries are drawn and the lines lay out. Everything prior feeds into it and everything after flows out of it. It is a period when normation is at a high and yet nothing has been established. Everyone is talking to everyone else and defining their own positions over and against those of others. Everyone is control of his own piece of the pie and no one owns the whole pie but everyone acts as if he does. Really understanding the second century literature is the only way to really understand Christian Origins. Christian Origins is not just about the creation of the NT books, or what the NT books can tell us about the first Christians. Christian Origins is about understanding how a messianic apocalyptic Jewish sect became a Christian Church by the time of Constantine.
5. If we take seriously the fact that the heretic was not a heretic before he was labeled a heretic by someone else, then the second century literature becomes even more interesting. What was the heretic before he became a heretic? Orthodox? Think of the Ebionites. Their form of Christianity was akin to the "original" form of Christianity of the Jerusalem Church (pre-Paul). By the mid-second century, they are heretics. Why? Because the Christian population became dominated by Gentiles, and their Jewish constituency became in their eyes an oddity and a liability. So the original form of Christianity was declared heretical by the newer Christians. What might this say about the Gnostics?