Just a note on the image thing first: You say you have long black hair and you don't know why people treat you like you have a goth or something image but they do, right? Have you tried to change your image? I can imagine that you maybe feel kind of like "well why should I change my image for other people" but that's just part of life sometimes. Sometimes you have to be wiling ot give a bit and fit people's expectations if you want something from them. SO if you want a job then sometimes you have to be willing to cut your hair and make an effort to conform to their expectations because basically it's you who wants something from them and not the other way around. They can sure as hell just go find someone they better like the look of. So anyway, you have to be willing ot give a bit sometimes. As for your girlfriend, why do you think she'd leave you just for your friend just because he has all this stuff? Surely she isn't that shallow is she? And what about him - you said he was seeing your gf's best friend? SO wouldn't she be splitting them up as well if that happened?
Have you spoken to her about this? Communication is so important within a relationship. Just speak to her about it and tell her how you feel. Tell her that you love her and you don't want to lose her and you're sorry you don't have all this shit but you really like being with her. Tell her that you're worried that because he has all this stuff she might be looking at him and comparing him to you unfavourably and ask her if that is actually the case.
So far it sounds like you're actually basing all this on assumption rather than actual communication and it's not us you should be speaking to about that - it's her. You should be talking to her about how much she means to you and how much you want to give her. And you should be showing her that you trust her by having that conversation with her. We tend to create these self-fulfilling prophecies for ourselves, so I wouldn't mind betting that if you carry on, based on these assumptions, thinking that she is going to leave you for your friend then she probably will because she'll pick up on those vibes from you, form that expectation and probably unconsciously fulfil it. It's just a funny thing that happens with people - they pick up your expectations and then go out and fulfil them,. SO the more you distrust her and the more you keep this to yourself the more I reckon you're actually unintentionally bringing it on.
So yeah, talk talk talk!
Good luck.