Tie pickling spice in a cheesecloth bag.
Combine spice bag, onion, and next 7 ingredients in a shallow dish or large heavy-duty zip-top plastic bag; add roast. Cover or seal, and chill 8 hours, turning occasionally.
Remove roast from marinade, reserving marinade.
Brown roast in hot oil in a Dutch oven. Remove roast, reserving drippings in pan. Whisk flour and sugar into reserved drippings, and cook over medium-high heat, whisking constantly, 2 minutes or until browned. Gradually stir in reserved marinade. Return roast to pan; bring mixture to a boil. Cover, reduce heat, and simmer 2 1/2 hours.
Discard spice bag and bay leaves. Remove roast, reserving drippings in pan; slice and keep warm.
Stir gingersnaps into reserved drippings in Dutch oven; simmer, stirring constantly, 2 minutes. Pour mixture through a colander into a bowl. Press vegetables through colander with back of a spoon. Serve gravy with roast.