The books that are now collectively referred to by scholars as the "Gnostic Gospels" are all later than the books that make up our present New Testament canon. The four canonical gospels, these being Mark, Matthew, Luke and John, all date from the mid to late 1st century CE; Mark was written right around the year 70 CE, with Matthew and Luke following approximately fifteen years later in 85-90 CE, and John coming last, probably in the late 90s or early 100s CE. All of the Gnostic gospels that we possess, including the Gospel of Judas, the Gospel of Mary, the Secret Gospel of John and so on date from the 2nd century and onwards. These Gnostic gospels are fraudulent (as your question asks) in the sense that they are pseudopigraphic, that is, they explicitly claim to be written by someone who did not actually write them. By contrast, all the gospels that exist in the New Testament were authored anonymously; the names now attributed to them came about as a result of early Christians trying to figure out who wrote the gospels that they were reading in their churches. So if you can imagine a group of early Christians sitting around reading what we now know as the Gospel of John, you can imagine them noticing the repeating theme in the text of the "disciple whom Jesus loved," or the "beloved disciple." This leads them to conclude that this beloved disciple must be the one who wrote the book; one of them has heard that John was Jesus' favorite disciple, so they conclude the book must have been authored by John. They then pass this conclusion on to other members of their church, who pass it on to other churches, and by such a process, the names of Mark, Matthew, Luke and John eventually became attributed to the canonical gospels. So while they attribution of names to the canonical gospels is generally held to be a long and honest (for the most part) inquest to ascertain their authors, the Gnostic Gospels were given their false attributions directly and intentionally by their real authors. While a more cynical perspective may view this as a deliberate attempt by the authors to deceive their readers into thinking the authors' views were really those of an apostle, it is also possible that the authors of the Gnostic Gospels were being sincere (to an extent). Gnostic Christians did in fact believe that Jesus had taught gnostic beliefs, and that he had confided secret, more advanced teachings to a favored disciple or apostle. So while the pseudonymous authors of the Gnostic gospels knew that their gospels were written by themselves and not by Mary or Judas, they may have sincerely believed that the sorts of things they were writing had in fact been taught to Mary or Judas by Jesus, and thus they weren't in fact being deceitful.

There is, however, one Gnostic Gospel that stands out from the others for biblical scholars, and this is the Gospel of Thomas. While nearly all scholars believe that this gospel reached its final form (the form we have today) in the 2nd century CE, many also believe that large parts of the gospel are derived from a much older source. The Gospel of Thomas is a collection of sayings attributed to Jesus, many of which mirror very closely those found in the canonical gospels, but in a simpler, more archaic form. This has led some scholars to argue that the bulk of the Gospel of Thomas, minus several distinctly gnostic sayings that were certainly added later, dates from much earlier than the 2nd century, possibly earlier than even Mark! While this is not universally agreed upon, and many scholars indeed firmly disagree, it is interesting regarding your question. However, to reiterate the actual gnostic portions of the Thomas gospel were added after the canonical gospels were written.

So to conclude, the Gnostic Gospels are all later than those of the New Testament, and are fraudulent in the sense that they are not written by the people they claim to be written by.