I just would like some input, Im writing a short paper on it and would love to hear what others think!
I think they reinforce another, not take away from each other. A clear example is the incredibily realsitic description of the upper room as it is described in the 5th section of the story. Faulkner gives incredible detail hitting on all of the senses to describe the dust and disuse of the room and the shocking finding of Homer in the bed, but the gothic/horror is made more so by the fine, realistic details such as the fact that Homer "lay in the attitude of an embrace" and the last detail, the long grey hair on the indented pillow next to him.
About the South, William Faulkner commented, "I love it; I hate it." The Southern gothic IS Faulkner. The Old South with its lovely charm turned to ruin--what can be more grotesque, and yet so realistic?
I would say no, neither the horror nor the closely related gothic elements take away from the realism in Faulkner's great story. In fact, I would say the opposite is true: the realistic details make the shock of the gothic more intense, and the two work together to generate the meaning of the story. Faulkner is showing us that in this sleepy, "normal" little town, there are intense hidden passions and grotesque crimes. By suggestion, every other house in Miss Emily's town might hide something similar. The gothic comments on the realistic elements.