I read some really old school Gothic Horror for a lit class in college. Here’s a list of what I can recall from that class. Though if you are open to new school recommendations, Mexican Gothic by Silvia Moreno-Garciais superbly chilling and really captures the old school vibe.

Frankenstein (1818) Which I assume you’ve read but if not is really good!

The Monk (1796) considered to be the OG gothic horror novel. It’s definitely a creepy one.

Carmilla (1872) Predates Dracula, girl power vamp.

The Turn of the Screw (1898) Madness or malignant spirits? Dear reader, this one’s up to you.

The Heart of Darkness (1899) Recently re-read for a book club. While it works being taught as a gothic novel, it’s actually way more powerful in the context which Conrad wrote it, a scathing criticism of European imperialism in the Congo. Recommend the Norton Critical edition to add the important context. Otherwise, it has the classic gothic genre themes of self vs other, familiar vs strange, civilization vs savage, human vs monster, flipping those notions all on their head (in beautifully written prose) like a brilliant gothic novel should.

I really enjoyed T Kingfisher’s The Twisted Ones (2019). According to the author, it’s based off of a story (The White People by Arthur Machen) that freaked Lovecraft out.

Really old school would probably be E A Poe. His stories stand the test of time better than others imo.