Ethan Frome doesn't have supernatural elements like some Gothic tales, but it features a strong sense of dread, and is very focused on death, and on the ways that people become trapped, both physically and mentally in their lives.
Novels that try to capture life as it was or is in a certain place and time are considered "Realist." Realist stories often deal with people who don't have much money or prospects. Sound like the people in Ethan Frome. This is where our theme "Technology and Modernization" comes in. The inhabitants of Starkfield are so poor in large part because the train no longer stops in their town. Starkfield and its inhabitants are being left behind by the modern world.
As such, this is also a Modernist novel. Modernists in Wharton's period were reacting, in their art, to new technologies and to the increasing industrialization of the world. Modernist art often focuses on the gaps in a story (go to out discussion of Ethan Frome's "Writing Style" for more), and is concerned with what happens when a character falls apart.
The love affair between Mattie and Ethan make this a romance, and the fact that it ended so badly make this a tragedy. The novella is careful to point out that even though nobody dies, everybody seems to think they would be better off if they had.