Slice the pumpkin, remove the seeds and all the internal filaments and bake, covered with a sheet of aluminum foil with holes for the steam to escape, in a preheated 350° oven for 40 minutes or until the pumpkin is tender.
Remove the skin and process the flesh in a food mill set to a large hole and let cool.
Combine the pumpkin flesh with the flour, 2/3 of the bread crumbs, 3 tablespoons of grated Parmigiano, eggs, salt and pepper and mix into a dough that is somewhat thick but also delicate and soft.
Divide the dough into three pieces, flour and, using the palms of your hands, shape them into long cylinders about one inch in diameter. Cut each cylinder crossways into pieces about 1 1/4 inches long and arrange the gnocchi on a lightly floured board.
Melt the butter in a pan over low heat, add the sage leaves torn into rough pieces and remove from the heat.
Meanwhile, cook the gnocchi very briefly in a large pot of salted water and as soon as they float, drain them with a slotted spoon, arrange in an ovenproof casserole, sprinkle with the remaining bread crumbs and Parmigiano, drizzle with the melted butter and sage and bake in a preheated 350° oven for 5 minutes.
Remove from the oven and serve immediately. Note: the amount of flour and bread required may vary depending on the moisture content of the pumpkin.