1. All non-canonical texts are Gnostic.
2. Canonical texts are early while non-canonical texts are late.
3. The non-canonical gospels are not "gospels".
4. The writers of non-canonical texts were hostile toward canonical texts.
5. Extant versions of non-canonical texts are their autographs.
The key thing is to avoid generalisations and to treat each document on its own merits and on its own terms. Then these fallacies can be avoided.