How do "A Rose For Emily" and “A Good Man is Hard to Find” explain or fit the description of Southern Gothic?
Southern Gothic fiction depicts a grotesque world in ruin. In the two stories you ask about, the post-Civil War South is in ruin.
"A Rose for Emily" is specifically Southern Gothic. Emily's home is a crumbling castle, the Southern economy is ruined, Emily's family is ruined, and the house's smell and Emily's penchant for holding on to corpses are grotesque. The town in the story is truly a world in ruin, and mystery, in the form of Emily, pervades the story.
"A Good Man is Hard to Find" is less obviously Southern Gothic, in that no house/castle is falling to pieces and no one sleeps with a corpse. A physical setting of decay does not center this work.
But the fictional world of "Good Man" is still a depraved, grotesque world in ruin. The grandmother is hideous, and creates a hideous world for those around her. She sees herself as a Christian, and sees her Christianity as a ticket to bully and criticize those around her who are not as godly as she is. She is unaccepting of everyone. The Misfit, of course, is grotesque in his violence, and presides over the ruined world of the story.
O'Connor's Southern world is a grotesque, ruined world, in which God's grace is at work, ironically, through the violent Misfit. It is an upside-down world in which the murderous Misfit is a more attractive character than the hypocritical, self-righteous grandmother.