The Gothic elements Harper Lee introduced into her novel, To Kill a Mockingbird--primarily centered around the Boo Radley subplot and the Radley house--add a bit of horror and mystery to the story prior to the Tom Robinson trial. Traditional elements of Gothic fiction (in this case, Southern Gothic) usually include portions of horror, romance and melodrama.

The effect of Gothic fiction feeds on a pleasing sort of terror, an extension of Romantic literacy pleasures... Melodrama and parody (including self-parody) were other long-standing features... the literary Gothic embodies an appreciation of the joys of extreme emotion, the thrills of fearfulness and awe inherent in the sublime, and a quest for atmosphere. 

Gothic fiction also includes a setting with ancient buildings, and in TKAM, the Radley house fills that requirement. All of the major elements of the genre are included. Jem, Scout and Dill thrill themselves with the thought of just getting a glimpse of the mysterious Boo. Passing by the Radley house each day on the way to school is an act of bravery to them. Touching the Radley house or setting foot on the property takes their courage a step further. The gifts from the knothole, the mended pants and the warming blanket around Scout's shoulders are exciting events to the children. The element of romance is even included with the budding childhood love between Dill and Scout. 

In To Kill a Mockingbird, Harper Lee does her best Southern-gothic Lite.  She's now Faulker or O'Connor, but here are the gothic elements present:

• a concern for the plight of the disenfranchised: blacks, social outcasts, women, the innocent, victims of racism, physical abuse (Tom Robinson, Mayella Ewell, Boo Radley, Dolphus Raymond)
• characters who are wounded or abused (Tom's arm, Jem's arm, Mayella's abuse, Tom's death)
• mysterious family history (Scout's mother's death, rumors of Boo killing his parents)
• mystery and intrigue at night (Nathan Radley shooting, Maudie's house fire, Bob Ewell's attack)
• haunted houses (Boo Radley's)
• mentally unstable or unstable women (the substance abuse of Mrs. Dubose)
• contrasting imagery, light vs. dark (Bob vs. Tom, Maudie's house fire at night, black church vs. white missionary society, divided courtroom)