This is a good question, and the answer is a bit ambiguous. An examination of representative definitions and criteria for Gothic literature (some of which I linked below) does not include an ending plot twist.
All of the criterion, however, point to things which make a surprise ending possible and even likely.
- Elements of suspense - omens, portents, and visions
- Supernatural elements - unexplained people, things, or occurrences
- Elements of terror - fear, violence, and the macabre
- Unnatural elements - illusions and hidden realities
While these and other criteria do not explicitly list the surprise ending as part of the elements of a Gothic tale, they also do not exclude it. In fact, many Gothic tales employ this plot device. Note Edgar Allan Poe's use of this literary technique in such Gothic works as "The Tell-Tale Heart," "The Black Cat," "The Pit and the Pendulum," and more. If the (arguable) master of the Gothic short story can use this device, it is certainly an acceptable and perhaps even a necessary element in the Gothic tale.
In short, the "unexpected plot twist" may be considered an "element of Gothic literature" in practice if not in the official (if there is such a thing) standards for this genre of literature.