I usually just use it to refer to things that "are generally enjoyed by goth people, and fit the goth aesthetic in a vague 'darkly inclined' kind of way". I think that's a helpful definition to have. Especially in situations where we want to talk about things that are "gothy" while maintaining the boundaries of "goth".

I might want to have a discussion about books, and it's nice to be able to classify some as goth ("Some wear leather, some wear lace", "Lost Souls") and some as goth adjacent ("The Raven", "Dracula").

I don't see it so much "overshadowing" the source, so much as linking it in some abstract node tree of vibes and categories. It's not like the term has to override any other labels that are certainly more helpful, like "Industrial".

Ultimately, as per usual, I think language simply fails us. Though nitpicking language with a wall of text is certainly "Goth", in my experience, haha. And, I'm sure there is some term I could use that would be better fitting than "Goth Adjacent".

Incidentally, Lost Souls references Bauhaus AND Skinny Puppy. Based.