Preheat oven to 350°. Cut the bacon crosswise into 1/4-inch-wide strips. In a 10- to 12-inch frying pan over medium-high heat, stir bacon until lightly browned and slightly crisp, 6 to 8 minutes. With a slotted spoon, transfer to paper towels to drain. Discard all but 2 tablespoons fat in pan.
Peel celery root and cut into about 1/2-inch cubes (see notes). In a 12- by 17-inch roasting pan, mix celery root and the 2 tablespoons reserved bacon fat. Bake for 30 minutes.
Meanwhile, rinse pork and pat dry. Sprinkle lightly all over with salt and pepper. Stir celery root, then set a rack above mixture (it can rest on pan rim). Set roast, fat side up, on rack.
Bake in the 350° oven until a thermometer inserted in center of thickest part of roast reaches 155°, 1 3/4 to 2 hours. Transfer roast to a platter or board and let rest in a warm place, 10 to 15 minutes. Meanwhile, stir bacon into celery root and return to oven; bake until celery root is lightly browned and tender when pierced, 8 to 15 minutes longer. Add salt and pepper to the mixture to taste. Wrap gingerbread rounds in foil and warm in oven, 8 to 10 minutes, then break into wedges.
Spoon celery root alongside pork roast. Garnish with orange slices and serve with Gingerbread Sauce and wedges.