Remove giblets and neck from cavity. Pull out any lumps of fat. Using a sharp fork, poke the skin of the goose all over. Dont pierce the meat, just the skin. Rinse goose inside and out; pat dry with absorbent paper. Sprinkle inside and out with salt, pepper, and 1 teaspoon ginger. Poke a few slits in the skin of the cavity. Place garlic slices into body cavity slits. Place the carrots, onion, and celery into the body cavity.
Tie the bird up. Place goose on rack, breast side up, in large roasting pan. Place the orange slices on top of the bird.
Roast goose 1 1/2 hours, basting with drippings. When the wings begin to brown, cover with aluminium foil. Turn goose over, breast side down. Cover the wings with aluminium foil. The roasting pan will have accumulated lots of fat; spoon 6 tbsp into a metal bowl and reserve.
Roast another 1 1/2 hours until a thermometer in the thickest part of thigh registers 175°F Let rest, covered, for 10 minutes.
Meanwhile prepare the pears so they can be roasted with the goose for one hour. Toss pears and lemon juice in large bowl. Pour 6 tablespoons goose fat into large baking dish. Place the pears in baking dish and toss with fat. Add sugar, 1/2 cup liqueur and remaining ginger to pears; toss.
Bake pears alongside goose until very tender and golden, about 1 hour. Spoon the pears into a serving dish with a slotted spoon to reserve as much liquid as possible.
Pour liquid into a saucepan, add stock, remaining liqueur. Bring to a simmer and reduce by half a cup. Sprinkle in flour while whisking and continue to cook a few more minutes so it is slightly thickened.