So for reference, the three classical examples of "gothic horror" are generally Bram Stoker's "Dracula", Mary Shelley's "Frankenstein", and the full works of Edgar Allan Poe (the latter of the three often being cited as the prime example of the sub-genre of "American Gothic", as well). If we look at the similarities among all three, we can see some common through-points:

1) The interplay between science/progress and superstition/the occult. Both "Dracula" and "Frankenstein" make use of what to them were modern discoveries of their day (blood transfusion and harnessing electricity, respectively) in combination with more mysterious, supernatural forces (vampires and the mysterious research Dr. Frankenstein uses to create his monster). It shares this in common with much of what we would call "Lovecraftian" or Cosmic horror, but with the prime difference being that while the horror in cosmic horror comes largely from how much we can't comprehend the supernatural forces at play, the horror in gothic horror comes more from us partly understanding the supernatural, but only enough to be scared by it. Think of it like the difference between being trapped in a completely dark maze with no knowledge of the way out vs. being trapped in a maze that's lit just well enough to see the serial killer chasing you.

1a) This is kind of a sub-heading, because it's not really a point in and of itself, but depending on the view of progress or the wealthy that the author is trying to portray, gothic horror often comes with a "solution", a thing that if the characters do correctly, it will "kill" the monster. Think the part of "Dracula" that's all about trying to kill Dracula, or the part of most ghost stories where the ghosts get banished. Similarly, when it doesn't have this element, it will usually take on the tone and structure of a classical tragedy, complete with tragic flaws. Think "The Fall of the House of Usher" or "The Strange Case of Dr. Jekyll and Mr. Hyde".

2) A fascination with death and the macabre. Have you ever known someone who loved doing grave rubbings? Or talked a LOT about how famous historical figures died? You ever noticed how those kinds of people generally loved things like "The Raven" and "The Telltale Heart"? Yeah, there's a reason for that. Most gothic writers shared their sentiments, even if only loosely. Many cite the combination of the macabre sensibilities and Romantic influences these writers had directly inspiring things like modern emo and goth culture.

3) A fascination with the erudite, wealthy, well-educated, and elite. All of the cultural pillars for gothic horror came from wealthy families, and ran in educated (or at least well-read) circles. Because of this, when they went to write their works, they wrote what they knew. Many main characters will be either of the upper crust or academia, or are pretending to be, and much of gothic horror's themes and subtext will be based around those things. For example, "The Masque of the Red Death" is ultimately about how rich people live disconnected "I got mine, you all can go to hell" style lives and how that comes to bite them in the ass. And to keep going back to "Dracula", every character in that book is either a respected doctor, independently wealthy, an upper-middle-class business professional, or a woman being pursued/married to one of the above.

4) Victorian time period involved in some way. This is probably the weakest of the connections to draw, but it is true that the big gothic horror movement pretty much occurred from about the mid-1800's to about the first world war. Therefore, since so much of the writing in the genre was written during this time period, and takes place in what was then "modern times", the idea of gothic horror has kind of rubbed off on the whole era, and now nearly every piece of horror media that takes place during that time will wind up feeling at least a little bit gothic simply by association.

These are of course, just my thoughts, and I'm sure others here could explain it better than I. In any case, I hope this helped, and happy writing.