Along with the death, decay, and dust of the houses and the aberrant psychological activity of the main characters, the mysterious and unnaturalness with which death is dealt with is Gothic in nature. For one thing, both Roderick and Emily still entertain ideas of their relatives and loves as existent even after their deaths.
I have to side with the other posters here. The elements of both death and decay are very important. In the same way, many Gothic tests have a sense of horror and the dark. Both texts speak to horrific elements (the entombing and the necrophilia).
I would want to focus on death and in particular the way in which one person is shown to cause the death of the other, perhaps out of motives of twisted love. On the one hand you have Roderick entombing his sister alive. On the other hand you have Miss Emily who is so determind to keep Barron with her that she becomes a necrophiliac, sleeping with his corpse after she has killed him.
Look at the characters in each story. Gothic literature often makes use of a physically or psychologically damaged characters. The Usher twins certainly have some mysterious maladies, and arguably, Emily is damaged in some way herself.
There is also physical and metaphorical decay in each story. Decay is a common gothic element. It can be distinguished by color, or by descriptions of rotten things or rotten deeds. The motif of decay can be either physical or moral.
One key Gothic aspect of both stories would be the houses. Both are mysterious and rarely visited; both contain dark and unexpected surprises; both are inhabited by persons who are also mysterious; and both are places where strange deaths occur.