Well, a great gothic novel is defined by its ability to create an atmosphere of unease and dread. Take 'The Castle of Otranto'. From the very beginning, the strange happenings in the castle draw the reader in. The best ones also play with the idea of the past haunting the present. In 'Rebecca', the memory of the dead Rebecca haunts the new Mrs. de Winter. And they often have elements of the romantic, but in a dark and twisted way, like in 'Wuthering Heights' where the love between Heathcliff and Catherine is as wild as the moors.
The best gothic novels often have certain elements. They usually have a spooky or mysterious setting like a haunted castle or an old, decrepit mansion. For example, in 'Dracula', Castle Dracula is a key element. They also deal with themes like the supernatural, death, and the unknown. Characters might be complex and often tormented, such as Victor Frankenstein in 'Frankenstein'.