Bring a large pot of generously salted water to a rolling boil. Add the jumbo pasta shells and cook for 8-9 minutes until al dente. Drain and rinse with cool water to stop the cooking process, then toss with a drizzle of olive oil to prevent sticking. Set aside on a clean kitchen towel.

In a medium mixing bowl, combine pumpkin puree, shredded gouda, ricotta, garlic powder, salt, and pepper. Stir until completely smooth and well incorporated.

Preheat oven to 350°F and lightly grease a 9x13-inch baking dish. Fill each cooled shell with about 2 tablespoons of the pumpkin-cheese mixture. Arrange the filled shells in a single layer in the prepared baking dish.

Make the brown butter sage sauce by melting butter in a light-colored saucepan over medium heat. Cook for 3-4 minutes until golden brown with a nutty aroma and brown specks form on the bottom. Remove from heat immediately, whisk in heavy cream, add fresh sage leaves, then return to low heat and simmer for 2-3 minutes until sauce coats the back of a spoon.

Pour half of the brown butter sage sauce over the stuffed shells and sprinkle with Parmesan cheese. Reserve remaining sauce for serving. Cover tightly with aluminum foil and bake for 25 minutes. Remove foil and bake for an additional 10 minutes until bubbly and golden. Let rest for 5 minutes, then drizzle with remaining sauce before serving.