Preheat your oven to 375°F.
Cook the jumbo pasta shells according to the package instructions until al dente. Drain and set aside to cool slightly.
In a mixing bowl, combine the pumpkin puree, Gouda cheese, ricotta cheese, Parmesan cheese, garlic powder, salt, black pepper, and nutmeg. Mix until well combined.
Stuff each cooked pasta shell with the pumpkin and cheese mixture and place them in a greased 9x13 inch baking dish.
In a saucepan over medium heat, melt the butter. Add the sage leaves and cook until the butter turns golden brown and the sage becomes crispy, about 3-4 minutes. Remove the sage leaves and set aside.
Add the heavy cream to the brown butter, stirring to combine. Cook for an additional 2-3 minutes until slightly thickened.
Pour the brown butter sage Alfredo sauce over the stuffed shells.
Cover the baking dish with aluminum foil and bake for 20 minutes. Remove the foil and bake for an additional 10 minutes until the cheese is bubbly and golden.
Serve hot, garnished with the crispy sage leaves.