Note that the GAU-8/A is on chambered up to 30mm. The MK19 is 40mm.
Second, the GAU-8/A is long-barrelled, and works via high repeating rate and high muzzle velocity. IOW it's for armored targets. The MK19 has a much shorter barrel, allowing high-explosive and gas rounds. We were talking mob defense, weren't we?
Third, the MK19 can employ indirect fire. So you can lob rounds over the defenses rather than rely on demolishing them.
Fourth, the MK19 is man-portable. The GAU-8/A needs to be mounted to a sturdy frame.