This is an excellent question, as both of these short stories are wonderful examples of how setting is used to build up the Gothic atmosphere and content that dominates these tales. In particular, what I would suggest you focus on is the way in which both the house and the wallpaper act as symbolic representations of the characters who feature in these texts.
As the anonymous narrator nears the somewhat foreboding House of Usher, he describes what he sees and his general feeling of unease in ways that we come to recognise as being a perfect description of the character of Roderick:
In this there was much that reminded me of the specious totality of old wood-work which has rotted for long years in some neglected vault, with no disturbance from the breath of the external air. Beyond this indication of extensive decay, however, the fabric gave little token of instability.
The house is defined by its "excessive antiquity," however at the same time it appears on the surface to be structurally sound, just as Roderick appears to be healthy and functioning as a normal human. However, just as the house has been rotting away for many centuries, we discover that the character of Roderick too is slowly disintegrating and unravelling.
In the same way, the narrator of "The Yellow Wallpaper" is a character who finds a strange parallel in the yellow wallpaper in her room and her own situation. The oppression and intellectual stifling that she herself is undergoing is something that is perfectly captured through the yellow wallpaper and the women or women that she sees trapped behind its bars:
And she is all the time trying to climb through. But nobody could climb through that pattern--it strangles so; I think that is why it has so many heads.
Setting is key to revealing character, and in particular the various pressures that face both Roderick and the narrator.