The point is the basis of heresy. That origin has nothing to do with the NT. So your assumption that the heresy was based upon what was not included in the canon is false. It had to do with them not being able to reconcile the "apparent" violence of the OT with the Good News. The Gnostic's error had nothing to do with the Good News. It had to do with their inability to interpret the OT. You literally have that same problem.Maybe you need to dig deeper? Contemporary scholarship largely agrees that Gnosticism has Jewish Christian origins, originating in the late first century AD in nonrabbinical Jewish sects and early Christian sects.