When the list of 208 titles is uploaded into Voyant, we can get a pretty good sense of the prevalent themes within the genre.
The first word cloud shows the data as-is, while the second shows the results once "romance," "tale," and "novel" are removed (these words are commonly used in subtitles). The word cloud hints that there will be a historical/medieval setting, possibly involving a castle or an abbey, and will probably have a mysterious tone.
On the other hand, I wanted to see if the texts themselves revealed anything about their contents. Along with our four key players, I included two additional works to round out our sample: Clara Reeve's The Old English Baron (1777), a literary successor to Otranto published twelve years later, and John William Polidori's The Vampyre, written at the same time as Frankenstein and published a year later. The works were uploaded to Voyant in chronological order so that we can see what changes (if any) occured over time.
[Note: If you hover over the "?" in the top right corner, and then click on the "Export a URL" button that looks like a square with an arrow coming out of it, you can open the dashboard below in a larger window.
Some findings:
The Mysteries of Udolpho is twice the size of the next largest book (The Monk) and nearly eight times larger than the shortest book (The Castle of Otranto). I had to remove the names "Emily," "Montoni," "St," "Aubert," and "Valancourt" from the word list because the frequency of these character names in Udolpho was affecting the overall results.
Dialogue seems to be used a lot more in pre-1790s novels (based on how often "said" occurs in the first two books). This could be a convention of the time rather than of any particular genre; we would have to compare our works to a sample of non-Gothic texts to explore this further.
The word cloud shows a lot of "sensory" words: "heard," "thought," "eyes," "heart," "hand." Many of these books involve characters navigating through mysterious circumstances or doubting their senses (as one does when confronted with a ghostly apparition). Whether this is a particularly Gothic pattern or a larger, era-specific convention, we would have to test against non-Genre works to find out more conclusively.