Preheat oven to 350° F. Place pumpkin in a shallow baking dish, skin side up, pour in 3 tablespoons water and bake until tender. Remove from the oven and cool. Peel off the skin and any burnt surfaces, then mash the flesh or put it through a food mill to yield 2 cups of pulp. (It's not recommended to use a blender or food processor, as the resulting puree tends to be watery and lacks body.)
Put pumpkin in a large bowl, season and begin to work in the flours. Use all the semolina and as much of the potato flour as gives a soft kneadable dough. Add salt, pepper, and nutmeg. Knead lightly until elastic. Cover and rest for 10 minutes. Break off little pieces about 3/4 inch long. Roll them quickly between the fingers to obtain a smoother surface, then press them with the thumb against the curved back of a fork or grater to get the traditional gnocchi shape. Dust lightly with potato flour and rest for 10 to 12 minutes.
Melt butter in a saucepan and cook it over a medium heat until golden brown. Keep warm. Cook the gnocchi, a few at a time, in boiling salted water. When they rise to the surface remove with a slotted spoon and transfer to warm bowls. Pour the butter over the top, sprinkle on some Parmesan and serve immediately. Extra Parmesan can be served separately.