Gothic is a an odd thing to describe. The word derives from the Goths, an East Germanic tribe that of course doesn’t exist anymore. And then there was the gothic movement, gothic architecture, gothic fashion, and bringing us closer to the present day, there is modern goth culture.
So what makes something gothic? Like many genres of stories, movies, and art, it’s not a simple question. Gothic is a collection of tropes. A common distinction would be period tales that have a darker spin to them. Or a style and look that mimics those periods of time. The original Frankenstein and Dracula novels (both of the 19th Century) in many ways set the tone for what would be considered gothic, especially in the horror genre.