The traditional religions define themselves as "not Gnostic" because the term is perceived as a marker for "heretic." So Mormonism, for one example, will self-define as "not Gnostic" (as we saw in some of your comments) while an outsider studying Mormonism and its formation may see the major signifiers of a modern Gnostic movement (as we also saw in some of your comments). Why? Because Mormonism has the esoteric teachings and practices surrounding the Temple in which the insider's knowledge is transmitted to the initiate, a transtheistic god viewed in ways very different from traditional Christianity, new revealed scriptures that reinterpret and critique traditional interpretation, and a critical subversive stance regarding traditional Christianity. And it became a separatist Christian tradition, if not a new religious movement (again, this is likely a matter of perspective).
Keep in mind that "gnosis" is not a particular set of beliefs so much as it is knowledge that is esoteric (hidden and revealed to a few), mystical (direct immediate experience of God), and subversive (critiques traditional religion).
It is this last segment of the definition that makes "Gnosis" different from other forms of knowledge and other religiosities. The Gnostics believe(d) that the traditional religion does not understand even its own scripture, and that they alone knew/know the true God who exists beyond the god(s) of traditional religion. So my question today is this. Are Gnostics "heretics" by self-definition? In other words, is this only a polemical perspective of the non-Gnostic defining the Gnostic as a heretic? Of is there something intrinsic about Gnosis that makes it heretical?