Preheat your oven to 375°F (190°C) and grease a 9x13 inch baking dish.

In a large pot of salted boiling water, cook the large pasta shells until al dente (8-10 minutes), then drain and cool.

In a mixing bowl, combine pumpkin puree, Gouda cheese, ricotta cheese, Parmesan cheese, garlic powder, nutmeg, fresh sage, olive oil, salt, and pepper, blending until smooth.

Stuff each cooled pasta shell with the pumpkin filling and place them seam side up in the greased dish.

In a saucepan, melt unsalted butter over medium heat, stirring until golden brown. Slowly whisk in heavy cream and sage, letting it simmer for 5 minutes.

Pour the Alfredo sauce over the stuffed shells, ensuring each shell is coated.

Cover the dish with foil and bake for 20 minutes; then remove foil and bake for an additional 10-15 minutes until bubbly and golden.

Let the baked shells cool slightly before serving, garnished with crispy sage leaves if desired.