Brown pork roast on all sides in a large skillet or under broiler to remove excess fat. Insert garlic slivers in pork and season with salt, pepper, sage, and thyme. Add broth and wine, if used. Cover and cook on LOW for 10 to 12 hours.
Remove roast and skim excess fat from juices; combine flour with about 1/4 cup of juices from the crockpot and whisk to smooth. Turn the crockpot to HIGH and stir in the flour mixture. Cook and stir until thickened (this may be done on the stovetop). Serve gravy over pork, with rice or potatoes.