The thing about Gerry being goth is that it makes an incredible amount of sense - not just as a spooky aesthetic, either. I think it has to do with the fact that he spent his entire life seeing and knowing the understructure of a world that is deeply horrifying to live in, especially if you aren't willing (as Mary was) to gamify it or align yourself with it enough to stop caring about the value of human life.
Imagine going to school at age 12, and being given brightly coloured books and told off for staying out too long on recess and pelted with dodgeballs during gym, all the normal middle school stuff - all the while knowing that 1: Everything you see around you is a thin facade for predatory forces of fear that could consume any person at any time, and 2: If you try to tell them they will either not believe you, or have their security and impression of existence irreparably altered and still not be any safer for it. Imagine feeling that misaligned with the reality every other person seems to be experiencing...and then looking in the mirror and seeing the most normal brownish hair, and the same bright clothes as any classmate, and the unassuming face of just anybody you might find on a playground, in a grocery store, anywhere. The face of the unknowing civilian, the face of a kid you never could have been, the face of potential prey.
Wouldn't that be a little bit unbearable? Would the pressure not grow until it claws at the inside of your chest, begging for if not a way to fix it or to tell anyone, at least some way to express how little the world you know matches up with the one you see?
I think, in that case, the alternative to running through the streets gripping random strangers by the shoulders and trying to make them realize just how tenuous and teetering on the edge of senseless terror and agony their existence is, is to go buy a big black coat and some shitty hair dye. Vent enough of the pressure that you can keep the rest to yourself. Find an equilibrium between two worlds that does not feel like a lie or an unacceptable vulnerability. "Goth." That's a thing people know. That's a thing that already exists in the world everyone else still lives in, but it's also a thing that doesn't shy away from morbidity, that copes with the doomed nature of existence by looking it in the eyes and saying Nothing here needs to be permanent to matter. I suspect it was Gerry's way of still interfacing with the world, to not cut himself off completely from the rest of unknowing humanity but to not present as an unbearably normal lie, either.
It just...adds. It's a foundational hint to who Gerry is and how he sees the world, so much more than just a quick visual aesthetic. It's one of my favourite choices re: canon character appearance expressed in the show, right next to Martin being "not the smallest guy".