The Gnostic Gospels — early Christian writings found at Nag Hammadi (and other sites) that reflect the Gnostic religious outlook — play the role of the earlier, more authentic, more female-friendly Christian scriptures in The Da Vinci Code. These early writings are fascinating and historically important, but they bear only the slightest resemblence to what Dan Brown describes.
Overview of Facts and Fiction in The Da Vinci Code
For more specific information on Mary Magdalene in the Gnostic Gospels, see the article Mary Magadalene in Ancient Texts.
Documents and Historical Evidence on the Gnostic Gospels
To evaluate whether the Nag Hammadi "scrolls" speak of Christ in human terms, all one has to do is read them. That can be done online here.
As to the dating of the Nag Hammadi texts, the manuscripts themselves date from about 350-400 AD. This is based on the datable papyrus used to thicken the leather bindings and the Coptic script. But these codices are believed to be Coptic translations of Greek texts, so the original texts would be significantly earlier.
Some Gnostic Gospels must date at least as early as the mid-2nd century, for the proto-orthodox bishop Ireneaus wrote in about 180 AD that the heretics "boast that they possess more gospels than there really are.'' (Irenaeus, Against Heresies)
The Nag Hammadi documents, though early, are probably all later than the New Testament gospels. One possible exception is the Gospel of Thomas. It was probably originally written around 140 AD, but some scholars think it records traditions dating from the 1st century. See the excerpts below for more information on this.
Secondary Sources on the Gnostic Gospels
Bart Ehrman, professor of early Christian history and author of Lost Scriptures, writes:
For critical historians, these documents provide valuable source material for understanding the milieus of Jesus and his early followers in the years after his death. But it is important to know what they tell us about those milieus. Misreading or misrepresenting ancient sources is as serious an error as overlooking them altogether. And as it turns out, Teabing makes several fundamental mistakes when assessing the importance of these modern archaeological discoveries.... Some of the Nag Hammadi documents... are Christian and do mention Jesus. Included in this collection are noncanonical gospels that appear to represent a Gnostic perspective. Far from portraying Christ as human, however, these documents are more interested in his divine qualities. (pp. 44-45) A number of the sayings of Jesus found in the Coptic Gospel of Thomas or in other writings of the Nag Hammadi Library have a definite Gnostic slant. The problem is that we have no evidence to suggest that Gnosticism could be found already in the first two decades of the first century - especially in rural Galilee. These Gnostic sayings must be later traditions, then, placed on Jesus' lips in some other context.... That is not to hold that all of Thomas's sayings need to be ruled out of court. Even in this Gospel, for example, Jesus tells the parable of the mustard seed, a parable also told (independently) by Mark. There is nothing particularly Gnostic about the saying, and it is found in two independent sources, one of which is very early. Conclusion? Jesus may well have spoken it. (p. 125)
In The Gnostic Gospels, Princeton scholar Elaine Pagels explains:About the dating of the manuscripts themselves there is little debate. Examination of the datable papyrus used to thicken the leather bindings, and of the Coptic script, place them c. A.D. 350-400. But scholars sharply disagree about the dating of the original texts. Some of them can hardly be later than c. A.D. 120-150, since Irenaeus, the orthodox Bishop of Lyons, writing C. 180, declares that heretics "boast that they possess more gospels than there really are,'' and complains that in his time such writings already have won wide circulation--from Gaul through Rome, Greece, and Asia Minor. Quispel and his collaborators, who first published the Gospel of Thomas, suggested the date of c. A.D. 140 for the original. Some reasoned that since these gospels were heretical, they must have been written later than the gospels of the New Testament, which are dated c. 60-110. But recently Professor Helmut Koester of Harvard University has suggested that the collection of sayings in the Gospel of Thomas, although compiled c. 140, may include some traditions even older than the gospels of the New Testament, "possibly as early as the second half of the first century" (50-100)--as early as, or earlier, than Mark, Matthew, Luke, and John. ...These diverse texts range, then, from secret gospels, poems, and quasi-philosophic descriptions of the origin of the universe, to myths, magic, and instructions for mystical practice.
Gnostic Society Library Lots of information and English translations of the Nag Hammadi Library and other Gnostic texts
The Gnostic Gospels: Introduction Online reprint from Elaine Pagels' book, on PBS Frontline, with info on the Nag Hammadi Library's dramatic discovery and contents
Matthew, Mark, Luke and... Thomas? Beliefnet interview with Elaine Pagels
Letting Mary Magdalene Speak Article by Karen King on the significance of the Gnostic texts, especially the Gospel of Mary
The Da Vinci Code's Shaky Foundation: Gnostic Texts Article by James Hitchcock for the Arlington Catholic Herald
Nag Hammadi Documents In-depth article by Jimmy Dunn for TourEgypt.net