Frankenstein was written by Mary Shelley on a dare. She and a few of her other friends were hanging out and they dared each other to write a good ghost story. The other folks were prominent writers as well, and Mary was the product of two successful writers. No one else took on the dare. At age 19, Shelley took this seriously and Frankenstein was born.
The book opens with a series of letters from a man on an adventure. Up in the Arctic region, he comes upon a man who tells the story of his upbringing. This story ends up including his creation of the monster, and then the monster ends up telling his story to his creator which his creator is re-telling.
Gothic horror is an off-shoot genre of the romantic era. Romanticism consisted of fantasy that could not come true. It was impossible. This scenario fits romanticism because man can only re-create man through one process. Romanticism also uses elements of nature to help portray mood or tone. This book is no different. In fact, nature most specifically builds suspense with it's light and dark connotations and the conditions of weather.
The idea that humans could do such great evil also contributes to the gothic nature of this piece. Up until the 1700s, much written text was produced by the church. During this new time, an author's imagination could run wild and did. Particularly because of what the French were able to accomplish in controlling other peoples, many had feelings about the ills of what man was capable of. The supernatural is indicative of this Gothic genre and Frankenstein certainly possesses that.