My wife had the History Channel on last night. It was on the Gnostic gospels. Very annoying. Just to get a sense of how incoherent some of these presentations are, they alternate between comments by Darrell Bock at Dallas Theological Seminary and mostly low level Gnostic groupees here and there. These people are on vastly different pages, yet the History Channel glides imperceptably from the comments of one to those of the other.
One particular sequence was the last straw. The narrator followed a path something like this:
1. The Gospel of Peter was not written by Peter.
2. The Gospel of Peter was a forgery.
3. What if Matthew, Mark, Luke, John were not written by them?
4. Would that mean they were forgeries too?
5. Would that mean the Gospel of Peter is the same as them and should be in the New Testament?
First, I'm not sure it is accurate to think of Gnostic gospels like the Gospel of Peter a forgery just because it wasn't written by Peter. We have many pseudonymous writings from the ancient world spanning centuries and it would seem to be a genre thing.
Further, Matthew, Mark, Luke, and John are anonymous. Their actual texts never mention their names. This is a big distinction. A person can have the most narrow understanding of inerrancy and not conclude any of these were written by their namesakes.
As for the question of what belongs in the New Testament, that is a completely different matter, one that ultimately is a matter of Christian reception of writings rather than authorship. I don't know of any Christian groups today who believe Jesus is Lord who receive the Gnostic writings as Scripture, so that's that.