The word "Gothic" originally referred to the Goths, Gothic novel carries a typical story focussed on the sufferings imposed on an innocent heroine by a cruel and lustful villian and made large quantities use of ghosts, mysterious dissappearances and other sensetional and supernatural occurances. The principle aim of any gothic novel is to evoke terror by exploiting mystery and veriety of horrors. So, story focussing on the sufferings of an innocent heroine by cruel villian, use of ghosts and supernatural events, arousing terror with horrorific situations are some of the most striking elements of gothic novel which can be applied in the Mary Shelley' novel too.
Frankenstein is a good example of a gothic novel which carries all above mentioned elements of gothic novel. The innocent heroine of the novel is Elizabeth Lavenza, Frankestein's wife. She is an innocent character. On the very day of the honeymoon of Victor Frankenstein and Elizabeth Lavenza the cruel monster kills her without any pity and sympathy towards her. So here monster has been presented as the villian of the protagonist's happiness. It is the most striking elements of any gothic novel which is completly applied in this novel; Frankenstein.
Similarly there is the use of ghosts, supernatural elements too. Crating a monster itself is a kind of mysterious circumstance of the novel. By the use of little known technology Victor gathers the bodily parts of his experiments he creates the monster. He uses the dead for his scientific experiments and out of the dead he creates the monster. It is very much supernatural and ghost like quality of the novel. The entire plot structure is dominated with the activities of the monster together with unbelievable supernaturally real miracles. At this point also the novel carries gothic elements.
The third essential element of any gothic novel is arousing terror with horrific situation. The novel carries this quality too. When there is the description about how the monster traveled in different place together with its destructive qualities it automatically creates horrific situation together with terror. For e.g. when the monster takes revenge against Frakenstein by killing the innocent heroine at the right day of their honeymoon there is very horrific situations filled with terror. There are other such situations too if we see the activities of monster. In this way Frankenstein possess almost all the qualities of gothic novel. So it is a gothic novel.