Not true at all, it is readily apparent to anyone with a solid knowledge of the Bible and Church History that the gnostic gospels are outside of the body of Christian teachings and reflect a philosophy that actually predates Christianity.
Well it would certainly make sense that gnostic teachings were outside the body of Christian teachings, since they existed BEFORE Christianity and its spinoffs arrived. More correctly then, it can be said that Christian teachings are outside the body of gnostic teachings. Logic compels it. They have as much right to say that their views are correct because of their history as any one else.