Preheat oven to 350 degrees F (175 degrees C). Spray a 9x13-inch pan with cooking spray.
Place porcini mushrooms in a bowl and add enough water to cover; soak until mushrooms have rehydrated. Drain water from mushrooms and reserve water. Chop mushrooms.
Cook sausage in a skillet over medium heat until no longer pink in the center, about 10 minutes. Cool slightly and slice on the diagonal.
Melt butter in a skillet over medium heat; cook and stir onion and garlic until onion is translucent, about 10 minutes. Add sage and nutmeg and stir. Transfer onion mixture to a bowl; add chopped mushrooms and sausage. Mix in reserved soaking water, pumpkin, Gruyere cheese, cream, salt, and pepper.
Bring a large pot of lightly salted water to a boil; add penne and cook, stirring occasionally, until tender yet firm to the bite, about 10 minutes. Drain and mix pasta into pumpkin sauce. Transfer mixture to the prepared pan and top with Parmesan cheese. Cover pan with aluminum foil.
Bake in the preheated oven until bubbling, about 30 minutes.
