Almost all of the gnostic texts date centuries later than the New Testament, with most scholars dating them to the late 2nd, 3rd, and 4th century. The only text that may have existed earlier is the Gospel of Thomas which may have dated to the late 1st to mid 2nd century. Outside of a very small group of scholars who make up the group called the 'Jesus Seminar', most scholars believe that the gnostics were exactly what they appear to be, a clearly heretical splinter group off of mainline Christianity that eventually died out. The Nag Hammadi writings are extremely interesting from a historicists point of view, but there's no reason to get excited about it from a faith or spiritual perspective.
Outside of tabloid-like sensationalism in History channel documentaries or DaVinci Code conspiracy theories, the writings are too late (and in many places, too flaky) to have any bearing on church doctrine. There are other apocrypha, similarly disregarded by the early church like The Gospel of Peter, which was obviously not written by Peter, but written in the late 2nd century. It has the cross that Jesus was crucified on bounding out of his tomb and talking. Another, is the Infancy Gospel of Thomas, which has Jesus as a child creating doves out of mud and killing a bully with the same curse he uses to wither the fig tree that produces no fruit.
A great book on the gnostic gospels and the recent excitement over them is professor Philip Jenkins' "Hidden Gospels: How the Search for Jesus Lost its Way". William Lane Craig also deals with Gnostic Gospels very well on his Reasonable Faith website.