In a large zip top bag, combine garlic powder, salt, pepper, mustard powder, cinnamon, cloves, allspice, ground ginger, oil, soy sauce, rice wine vinegar, shaoxing wine and honey. Add brisket. Press out as much air as possible, seal tightly, and refrigerate 6-8 hours, turning occasionally.
Place brisket in slow cooker, and empty marinade over it. Cook on low 6-8 hours.
Remove brisket to a plate, tent with foil and let rest 10 minutes. Meanwhile, skim off as much fat as possible from juices in slow cooker. Turn slow cooker up to high. Dissolve cornstarch in cold water, and drizzle into defatted juices. Stir until thickened.
Slice brisket thinly, across the grain. Serve with gravy.