Remove excess fat from meat. If necessary, cut meat to fit into a 3 1/2- or 4-quart slow cooker. Place meat in cooker.
In a medium bowl, combine 2 cups water, ketchup, soup mix, Worcestershire sauce, cinnamon, garlic, and pepper. Pour over brisket.
Cover and cook on low-heat setting for 10 to 11 hours (or on high-heat setting for 5 to 5 1/2 hours).
Remove meat to a serving platter, reserving cooking liquid. Cover meat with foil to keep warm.
For gravy, pour cooking liquid into a glass measuring cup. Skim off fat. Measure 1 1/2 cups cooking liquid, set aside (discard any remaining liquid). In a small saucepan, stir the 1/4 cup cold water into the flour. Stir in reserved 1 1/2 cups liquid. Cook and stir over medium heat until thickened. Cook and stir for an additional minute. Slice beef across the grain into thin slices. Serve with the hot gravy.