I haven't made any customizations within GSX, and I don't believe I've made any modifications to TJSJ - I'm away from my sim PC so I can't immediately confirm that, however. Certainly nothing involving vehicle textures, that I'm aware of.
I generally only modify individual gates to suppress the air stairs, particularly at a handful of older airports that have modeled (often static) jetways that GSX obviously can't see. (FlyTampa KBUF comes to mind, as an example. I've just been too lazy to "fix" it with GSX Level 2.)