Place potatoes, onion, and garlic in a large pot. Add enough water to cover. Season with salt and pepper and bring to a boil over medium-high heat. Reduce heat to medium-low and simmer until potatoes tender, 15 to 20 minutes.
While potatoes are cooking, heat a large skillet over medium-high heat. Cook and stir ground beef in the hot skillet until browned and crumbly, 5 to 7 minutes. Drain and discard grease.
Add beef to the stew mixture; cook over medium heat for 15 minutes.
Combine 2/3 cup water and gravy mix in a small bowl. Stir into the stew very slowly. Let simmer until thickened, about 20 minutes more. Taste and adjust salt and pepper.