No wall is perfectly straight. There will be areas where small gaps are visible. The size of these gaps is determined by the severity of the bows in the walls. Granite is extremely rigid and cannot bend to follow the irregularities of walls.
A small gap does not affect your countertop performance and is recognized as acceptable by the Natural Stone Institute. Countertops will not be replaced due to gaps between the walls and the countertops. Installing backsplash can minimize the appearance of gaps and caulk, painted to match the walls will make the appearance of a gap virtually disappear.