It might not feel like a high priority to look Goth, but if you are heading directly to meet up with someone and you want to not look travel gross, it might be a priority for you. The key issues of flying for Goths are balancing looking Goth, going through security, and being comfortable.
For the most part, TSA security do not really care about your unusual fashion. To be honest, they've probably seen stranger. As long as you aren't breaking in flight rules, which you can check on your airline's website, you'll probably be fine.
First let's consider going through security. Body piercings are usually designed with non-magnetic metals which will not set off metal detectors, but keep the amount of metal in the rest of your outfit to a minimum if you aren't sure. Garments decked out in badges, safety pins, spikes, and buckles all have the potential to set off metal detectors.
Also keep in mind that TSA may ask you to remove accessories like jackets, belts, scarves, etc. so that they can pass you through security quicker. To make sure you aren't bogged down by having to take these on and off, keep your outfit simple. If you wear petticoats and don't want to pack them, be aware that TSA may either perform a pat-down or a search to ensure you aren't hiding anything in them. If you would be bothered by this, read up on your privacy rights and consider packing the petticoats instead.
To stay comfortable, don't wear clothing that is too tight or structured. If you wear skirts, this is a great time to bust one out for comfort's sake. If not, avoid anything too tight or uncomfortable. Breathable materials rather than polyester are a good idea. Lastly, although this sounds silly, make sure your shoes fit. Feet and hands tend to swell at high altitudes, and you don't want already tight shoes to become pinchy.
As far as makeup goes, you may choose to wear some on the plane but expect to need a touch-up if your flight is over three hours long. Personally I wouldn't wear anything elaborate for a plane journey just because the touch up would take so long, but I would recommend keeping a chapstick of some kind in your bag to counteract grossly drying airplane air. Just keep the liquid and cream makeups in a clear plastic bag for passing through security and you should be good to go.
Personally today I'm wearing a comfy black knee-length dress from Forever21, a borrowed hoodie, polka-dotted socks, and my black slip ons from Payless.
What do you wear to fly?