The simplest answer to this question is that Gothic fashion that Goths wear... But that's about as helpful identifying a brown, sticky thing as a stick.; so it bears some consideration. In the minds of the general public, Goth fashion is the funeral parade of clothing worn by moping teenagers in town centres on a Saturday afternoon; another simplistic vision, one created by a hostile world that's keen to ridicule anything that's different.
In truth defining Goth fashion is as difficult as finding a single sound and declaring it epitomises Goth music: there are too many strands to do that easily. Beyond a few basic truths, there aren't many keynotes that bind the fashion, or indeed the subculture, together. It's another blanket statement, giving no real indication of what the clothes Goths wear actually look like, and in truth that's probably for the best. A gathering of Goths can be as unremarkable as a group of people wearing black tee shirts and tight trousers, a legacy of Punk the tribe's never quite let go, or it can resemble a fantastical funeral parade for a vampire pirate or Victorian mad scientist (or any number of unlikely combinations) replete with straitjackets, immense boots and more ruffles than you can shake a stick at.
Where does the fashion of the various strands of Goth come from?
If we generalise we can certainly see a shift away from the mainstream's idea of what makes good clothing and an embracing of both femininity and androgyny for both sexes. There's a focus on tactile, sensuous fabrics like velvet, satin, lace and, in some cases, more outre materials like PVC or rubber.I suppose the classic, basic, look for men is comprised of drainpipe jeans and a tight top (the equivalent being a long black skirt and tight tee shirt for women) but as I say this is the very base of the fashion. It leads quickly to more flamboyant clothes because, well, they're more interesting. It's here that we start to find the corsets, the immense dresses, pirate shirts, kilts, fetish gear and other items of desire. Things get more confusing when you consider the bright influences that bled through from the rave scene in the 1990s, epitomised by the Cyber Goth scene that's so focused in Paris. This strand of Goth clothing brings in brighter shades of colour from Dance culture but meshed it with the cuts and styles Goth had already embraced, and the 'no future' attitude that epitomised both Punk and Goth. History has been plundered repeatedly, period fashions and motifs are reinterpreted for modern times, always with an eye on the dark and the sensual, something that Steampunk has adopted as well. In other places, particularly America, television has been a huge influence, inspiration has come from shows like Elvira, the Addams Family, or Dark Shadows, these last both recreated as films in the last twenty years, as well as the pantomime horror antics of bands like Alice Cooper, and The Misfits. That's before we even consider the way influences from Japan in the form of Gothic Lolita and anime. What Goth fashion does is sweep up all these influences and set about reinterpreting them into something else. This tendency is both a weakness and a strength; Goth has grown diverse over the past thirty years, without necessarily becoming more dilute. The core of it has remained strong and supple, probably because the heart of the tribe is devoted to an emotion and a way of expression rather than a specific sound or look; those are mutable and changeable. The weakness lies in the fact that it makes it difficult to pin down anything for sure about Goth; if two people express themselves in largely different ways, drawing from different influences then to an outsider it makes the picture of what's going on more complicated.
As with the vast majority of subcultures, there is inevitably the issue of authenticity. If anything is despised more than a poser or pseudo-goth I'm not sure what it is and this applies to fashion as much as anything else. A true Goth look is not simply bought, it is sought, pieced together and designed, consciously or unconsciously as a form of expression. It is as much a way of creating art as it is anything else.