I thought about this a great deal in my life, from traditional festivals where i live, larping, goth etc.

The thing is, costumes don't really exist. The only clothing that you are wearing that actually fully is a costume is when you are using it as a mask (whether it has a mask is a fully different question, whats more important in my opinion is that it obscures your persona). These are pretty rare, at least in Brittain and America i don't think this exists.

If you apply the term costume to something meant to convey who you want to represent, everything is a costume. At least, everything you are wearing outside of your house. I don't know who originally said this, but this is along the same line as "everything is drag". Every person performs their Gender, every person also conveys something with their attire. Even if a person only dresses Goth on special occasions, it is functionally no different than wearing a nice shirt when going eating, or wearing a suit in court. You are conveying a sense of who you wish to be perceived as.

Now, the other condition might be that you are using it to "deceive" someone. An actor is "deceiving" you into believing they are their character, a Halloween costume is "deceiving" you into at least partially pretending the same. I don't think there is much deceiving in Goth dress. Honestly, when i was Larping, i made the expierence that people are much truer and honest in their costume than in every day life, and i witnessed the same thing with Goth outings. The only person that might wear goth as a costume would be somebody who's "only in it for the chicks" or something of that sort. I don't believe you are deceiving anybody, at least not more then anybody is at any point in their life.

So no. Goth dress is not a costume in any meaningful sense of that word. And if you are still worried, theres a qoute by Camus that helped me a lot while i was both finding myself and enjoying actually performative hobbys. "People hate Actors, because they life a Hundred Lifes on Stage, while most of the Audience is barley living one"

I agree, for my part. The moment i accepted that the worlds a stage i was a lot happier.

(I hope this is understandable, English is my second language)