There are many people out there whom claim to be 'Gothic', but what does that really mean? Are their really any rules that a true 'Goth' lives by? I think not. In my humble opinion, the only thing that pertains to any supreme Goth of pure truth is 'creativity' and 'originality'. Those two words may sound alike, but really they aren't.
People are attracted to the Goth subculture for all sorts of reasons. Some are there because of their dark and brooding personalities, some because they really like the music, some because they like the clothes. I suspect that all of the above figure to some extent or other in most Goths. In order to show that one reason is somehow more authentic than the others, or that some Goths are more truly gothic than others, you'll have to import your criterion of selection from outside the gothic subculture.
For to the extent that there's an immanent criterion of being a Goth, it's a fairly flexible criterion, one that won't affect the radical purge of the gothic subculture of which you seem oddly desirous. And I should think that the homogenization of the gothic subculture that would result from the application of such an authenticity test would be the death of the subculture, a death that is in no interesting way gothic.
I think it's very true that Gothic love is a label often applied from the outside rather than from within. Part of it is because the Goth subculture tends towards valuing individuality (despite the outward appearance the black dress code sometimes conveys) and so tends to attract people who tend to reject labels as too simplistic.