You're overstating. The gospels themselves do not claim to be written by Matthew, Mark, Luke, or John, neither are they signed. Those names were assigned by tradition. We do not know who wrote the gospels; that doesn't mean that the use of traditional names makes them "forgeries." In addition, it was customary for centuries to write a book in someone else's name (see, e.g., chapters 40-66 of the Book of Isaiah). The practice is called pseudepigraphy and was meant to either carry on a great teacher's tradition or to honor that teacher. See, for example, the pastoral epistles, which were not written by Paul, but written in his name.