To make the gravy, remove the meat and keep warm. Pour the hot liquid from the Crock-Pot into a saucepan. Stir in the crushed gingersnaps and brown sugar. Simmer on low. Stir in the sour cream with a whisk until blended. Add flour to thicken. Simmer. If the sauce is too thick, thin with water. Too thin, add a pinch of flour until it is the desired consistency.