Some of the history, starting with The Castle of Otranto, followed by a gap, and then a sudden deluge of imitiators, as if everyone had just discovered the possibility of imitation. (Rather like the gap between The Lord of the Ring, and its imitators.) Enormous masses of literature that vanished without a trace. Heck, for a good long time the works that Catherine and Isabelle chattered about in Northanger Abbey were thought to be her inventions; they were, in fact, works in existence.
It was called Gothic as an insult, just as the Gothic cathedrals were. It was, after all, not in the pure classical vein that the Enlightenment admired beyond all bounds of sanity. And it was all part of the emotional reaction to what was praised as reason. Just as the latter burst, at the end of the Victorian era, was a reaction to the science and industry of its time.
Nowadays, it's an academic euphemism. One talks about Gothic when one does not want to admit to be talking about horror.
One panelist gravely described the original Gothic as the origin of all modern-day genres. It's got your romance, it's got your horror tropes, it can have fantastic elements, it's centered around a mystery.
Northanger Abbey was strongly recommended.