Remove any silvery-looking skin from the pork. In a medium bowl combine garlic, ginger, sugar, soy sauce, hoisin sauce, sherry and ketchup. Divide sauce in half. Place pork in a shallow dish and brush liberally on all sides with half the sauce. Cover and refrigerate for at least 4 hours or overnight. Cover and refrigerate the remaining sauce.
Preheat barbecue grill to medium. Remove pork from marinade, discarding marinade. Place pork on greased grill and grill for 30 to 35 minutes, turning occasionally, until a meat thermometer inserted in the thickest part registers 155º and just a hint of pink remains inside. Transfer to cutting board, cover loosely with foil and let rest for 10 minutes. (The pork will continue to cook in its own heat and the temperature will rise by about another 5º.) Warm the reserved sauce in the microwave for about 20 seconds. Thinly slice pork and serve with sauce.
To cook this pork in the oven, place pork on a rimmed baking sheet lined with greased foil and bake at 375º for 30 to 40 minutes or until a meat thermometer inserted in the thickest part is 155º and just a hint of pink remains inside.