Edgar Allan Poe is considered a Gothic writer primarily because he explores themes related to the certainty of death and the darker aspects of the human experience. His works often feature elements of horror, madness, the supernatural, and the macabre, which are hallmarks of Gothic literature.
Although he does incorporate personal experiences and emotional tensions, the defining characteristic of Gothically themed literature is its focus on the psychological state of characters, existential dread, and the fear of the unknown, all of which Poe weaves into his stories and poems. His attention to death, decay, and the eerie atmospheres in his work strongly aligns him with the Gothic tradition.
So, while he touches on various themes, it is his exploration of the certainty of death and the exploration of fear that solidifies his position as a key figure in Gothic literature.