Trim the rump roast of all excess fat.

In a large bowl, combine red wine, cider vinegar, sliced onions, sliced celery, whole allspice, whole garlic cloves, salt, and pepper; stir well to create the marinade.

Place the roast in the marinade with the fat side down. Ensure the roast is fully submerged.

Refrigerate the roast in the marinade overnight, allowing flavors to penetrate.

Pour the vegetable marinade into the crock pot.

Place the marinated roast in the crock pot with the fat side up.

Cover the crock pot and cook on low setting for 8 to 12 hours, or until the roast is very tender.

Thirty minutes before serving, remove the roast from the crock pot and turn the setting to high.

In a separate small bowl, make a smooth paste of flour and water.

Stir the flour and water paste into the crock pot with the crushed gingersnap cookie crumbs.

Cook and stir until the gravy thickens to your desired consistency.

Slice the roast and return it to the gravy in the crock pot for serving.

Serve hot with the gravy.