Preheat the oven to 350 degrees F (175 degrees C).

Place roast in a roasting pan. Arrange carrots, celery, and garlic around the roast.

Whisk brown gravy mix and water together in a small bowl until smooth. Stir in onion soup mix, condensed cream of mushroom soup, and cola. Pour mixture over the roast.

Cover the roasting pan and bake for 1 hour in the preheated oven.

Reduce oven temperature to 225 degrees F (110 degrees C). Continue baking for 2 hours. Remove the roasting pan from the oven and carefully turn the roast over so it is coated with the gravy. Cover and return to the oven until the roast is fork-tender, at least 2 more hours.

Transfer the roast to a cutting board and let rest for 10 minutes before slicing. Serve with the vegetables and gravy.