Fashion can be a symbol of resistance and rebellion, of rejecting mainstream values, but sometimes that symbolism is a projection from those around the wearer, not the wearer themselves. Sometimes it is from the wearer - hippies utilised this as a tool by wearing clothes they made themselves or made from costumes to be walking symbols of what they believed in, wearing CND logos (Peace symbols), natural designs, or vivid, almost lurid colours full of vibrancy and conveying an experiential zest for life. Punk, the subculture that is Goth's antecedent, became stylistically manifest in deliberately ripped and tattered clothes, a D.I.Y ethos and unnatural hair colours and fierce styles that made the wearer immediately visible as a Punk and thus making the wearer carry everything that such a style stands for. Punk style started as a visual manifestation of an anarchist attitude, of a D.I.Y ethos that rejected the consumerist materialism of high fashion, of colourful and and sometimes painful self expression, of being oneself at any cost, even if over the decades, like Goth, it has slowly become something that is bought and copied. Goth inherited a lot from that, especially styles like Deathrocker.
Anyone going out in public in the complete regalia of any subculture is marking themselves as visibly outside the mainstream, as visibly part of a different 'tribe'. Goth is no exception, and if anything, less socially acceptable than being a Hippie or Emo, as Hippie has gradually become accepted as an extension of being "a bit Bohemian" and Emo fashion in terms of actual garments, is more a darker turn on mainstream garments such as hoodies and skinny trousers than something wildly different.
If a Goth walks down the street in leather trousers, knee-high stompy boots, a leather corset with steel bones and steel studs, a short-sleeved shirt and torn fishnets for an under-shirt, with spiked cuffs adorning their neck and wrists, leather fingerless gloves adorned with even more studs, hair dyed black and purple and wildly styled, face pale and painted up with black designs that accentuate their eyes, and with ears and face full of metalwork, that Goth is expressing their inner beliefs and selves externally; one that to a degree, because it is so removed from mainstream fashion, includes a certain amount of defiantly doing your own thing. They become immediately visible by how different they look to the majority of the people around them, and while they are probably not dressed that way to garner attention, they know they will receive it. A lot of the attention they will get will be negative, and sadly a lot of young Goths eventually give up the subculture because they do not want to face the harassment and sometimes violence that will be heaped upon them for their choices.
While Goth is not something to take up in order to rebel, anyone who is a Goth is innately going to be seen as a rebel because they are, with varying levels of deliberation, taking the alternative path and not choosing the mainstream, popular options. It may not be the active rebellion of Punk, but it as an act of separation. Even if it is merely a rejection of mainstream fashions, aesthetics and music, it is still an act of separation. It's not always a case of drawing a line in the sand to say "I am not like them", but by looking different, it often becomes regarded as such from those outside of the subculture. Goth is a path where those who live it tend to end up walking a very self-determined road, something inherited from its punk ancestors. Personally, my rejection of the mainstream goes further than a rejection of mainstream fashion, aesthetics and music. I am opposed to to quite a few aspects of modern society that have become all-prevalent and which I find insidious, worrying and are not something I wish to be part of, but how much individual Goths reject mainstream values really depends a lot on the individual Goth; a lot of us tend to be quite strong on the idea of people thinking for themselves and not defaulting to what is popular or given media prevalence, but other than that Goth doesn't really come with any ideology beyond an appreciation for the darker things in life.
Goth is not a political movement, and so it does not carry the same anarchic connotations as punk, or the same pacifist, environmentalist and psychedelic connotations of the Hippies, if anything it is a philosophical or artistic movement that seeks beauty in the darkness, as inspired originally by the songs of certain bands, but with a history that draws on creativity from many periods. Goth is not about railing energetically against mainstream society, the systems of authority, or the many injustices of the world, but it does have that strong individualist ethos within it. Goth is also about looking in dark places, and if you look in the dark corners of the world, you will be moved by what you see there. Each Goth will believe their own creed, politics, spirituality and vision of the world, and generally allow other Goths, and other people in general to also have their own opinions - that said, Goths can turn political, philosophical and religious debate into a flame-war or argument too, as we are after all a collection of strong personalities that will sometimes clash, and no group of people is perfect - but overall, we seem to have too strong a sense of individuality to all come with a similar set of opinions.
Goths tend to see their clothes as part of their identity, an outward manifestation of their inner selves as much as an identifier of membership of the subculture. The clothes we wear within the broad umbrella of Goth are hugely diverse, especially as, despite the fact there are now goth 'brands', that there is a huge creative element that hand-crafts, hand-sews, modifies and designs things for themselves, and those clothes will often be very carefully thought out before being worn, dressing to match the person's mood, their self image, the the values they hold, the place they are going to. Goths do not tend to wear something to "fit in", nor do they generally wear things because they are popular amongst other Goths (although in the age of the internet there has been an increase in 'trends' within Goth - a couple years ago it was wearing horns, probably inspired be Maleficent, then it was wearing buckled and studded 'harness' inspired straps, currently it's the "witchy" or "occult" trend, which I have strong opinions on, etc.). Goths, while often dressing in sexualised ways, tend to do this to express their own sexuality rather than to pander to someone else's.
Goth fashion can be highly gendered, but at the same time, gender-bending, androgyny and transvestitism are generally accepted within the Goth community. Men can wear skirts and make-up without the assumption amongst other Goths that they are doing so because they are gay, and they can also do so in ways that are not traditionally un-masculine - for example wearing a Rivethead utility kilt/skirt and black eyeliner and black nail-polish, in an outfit clearly inspired by dystopian science fiction rather than by drag queens. The Goth community tends to towards equal standards and expectations of behaviour amongst gender; women are encouraged to be strong, men are allowed to be emotional. The confidence gained from outwardly flouting gender norms in terms of dress within the Goth community often translates to a confidence to continue doing so in a wider context.
As Goth encourages visible self expression through fashion, piercings, tattooing and other personal, aesthetic avenues, it fosters an atmosphere where people are proud to be themselves, something at the core of every civil rights movement. It also allows people who are trying to be the change they want to see in the world to be a visual manifestation or representation of that change, in the way of being a walking work of art. Also, once someone is visually different, that often gives them the confidence to be behaviourally different - it is often the start of acting according to one's true self rather than according to the expectations of mainstream society as once someone is visually part of the "other" then they feel freed of the constraints of being part of the norm. This though, is not linear cause and effect, as it takes a certain amount of confidence to so visibly reject the norm, as the mainstream can be particularly cruel over things as seemingly unimportant as clothes, because clothes are a manifestation of the inner self, and are therefore some of those in the mainstream act out against people they think do more than look different (otherwise 'edgy' hipsters misappropriating Goth would get the same harassment Goths do) especially if there are prejudices and assumptions about the specific kind of "looking different". Being visually different does not necessarily mean being as radically different as taking on a subculture such as Goth, Punk, Hippie or Lolita, or even dressing in a way culturally attributed to another gender, it can be as simple as wearing clothes that belong to the mainstream, but are ones that you like from a few years ago, and not updating your wardrobe every six months because last season's clothes are out of fashion, or simply dressing in less gendered or less sexualised way.
If you approach clothes, personal aesthetics and style from the standpoint of using it as a form of self expression, as a manifestation of the inner self and ones tastes, rather than from the standpoint of worrying about trends, "being fashionable" or from the standpoint of trying to fit in, look "normal" or caring about what others think of your appearance, then you become freed from the effects of fashion advertising. You cease to think in ways that most advertising targets - adverts generally work by trying to sell the product as being part of a lifestyle, or by targeting insecurities, and once you are no longer interested in those lifestyles and no longer have those insecurities, you begin to see through advertising. If your beauty ideals are based on your own tastes rather than on the appearances of celebrities and models, then the products those celebrities and models endorse no longer seem better by association. This does not necessarily entail being Goth, but it does entail thinking for oneself, which is something Goths tend to be good at - I'm probably going around in circles in saying we tend to be ardently individualist! A lot of people who aren't Goth tend to be good at it too, though, and being Goth is by no means a mark of superiority above "sheeple" masses.
Once you see clothes, personal aesthetics and style from the standpoint of self expression, and tend towards creativity, then it provides a way of being a walking artwork, from designing clothes to designing tattoos and everything in between. As clothes are an outward manifestation of my inner self, I'd rather make that statement myself instead of mass-produced designs. This does not necessarily mean making clothes from scratch, but includes modifying things and customising them. Having a broad choice - and the main chains and high-street stores do actually sell a good variety - is not the same as getting your own vision fulfilled, and that is where making and customising has its greatest strength: you are in control of the project. Yes, there are still the limitations of findings and materials, although even those can be overcome, albeit sometimes at quite a cost, but there is still so much more scope for fulfilling personal vision.
I'm probably going to write at length in future about how my clothes express my inner self, and about some of the choices I make - for now, here's ::a post:: from a while back that covers that to some degree (and in general, don't be afraid to read and comment on older posts; I do still get informed about comments, and I do tend to reply!)