Based on the conventions of the Gothic horror tale, how can Frankenstein be considered a gothic tale?
There are plenty of aspects that make up Gothic horror tales, and a large number of these can easily be found in this excellent novel, such as the presence of fear and terror, ambiguity concerning the identity of the creature, scientific discoveries going beyond the realm of acceptable knowledge and murder and violence. However, one Gothic ingredient of this excellent novel is the way in which setting is used. Gothic fiction is famous for the way in which it uses settings that are extremely remote and isolated. These settings make the characters themselves isolated and cut them off from fellow men, allowing the psychological nature of Gothic literature to be explored more fully.
This novel is replete with such landscapes. Consider Frankenstein's meeting with his creature on top of a mountain, his eventual pursuit of him to the Arctic and the remote nature of Victor's family home. Likewise, Victor picks one of the remotest locations on the British Isles to make a companion for the creature:
With this resolution I traversed the northern highlands and fixed on one of the remotest of the Orkneys as the scene of my labours. It was a place fitted for such a work, being hardly more than a rock whose high sides were continually beaten upon by the waves. The soil was barren, scarcely affording pasture for a few miserable cows, and oatmeal for its inhabitants, which consisted of five persons whose gaunt and scraggy limbs gave tokens of their miserable fare. Vegetables and bread, when they indulged in such luxuries, and even fresh water, was to be procured from the mainland, which was about five miles distant.
Note the extreme isolation of this island, which will be of course used for the most horrific of tasks. It is the "remotest" of the Orkney islands and is beaten by the elements. The mainland is five miles away. Such a setting is entirely appropriate for the novel as so much of it operates on the border of the imagination. By placing scenes in such extreme environments, Shelley makes this as much of a psychological novel as a horror novel.