Place the marinade ingredients in a Ziploc bag with the meat, seal tightly and refrigerate for about 24 hrs.
After marinating meat, place meat with marinade and beef broth, celery, carrots, and horseradish in a crock-pot to cook for approx. 10-12 hours on LOW.
Strain marinade sauce into 2 qt. saucepan. Add flour (with water to make a paste) and gingersnaps to thicken. Bring to a boil, lower and simmer for approx. 5 minutes. Stir constantly until thickened. Strain into gravy boat.