Preheat the oven to 400 degrees F (200 degrees C). Line a rimmed baking sheet with parchment paper.
For the topping, melt butter in a pan over medium-low heat. Cook, stirring often, until the milk solids turn golden brown and the butter smells nutty, 10 to 12 minutes. Pour butter and any browned bits into a small bowl and allow to cool to room temperature. Whisk in the heavy cream and set aside.
For the scones, whisk flour, brown sugar, baking powder, salt, and garlic powder together in a large mixing bowl. Add grated butter and work into the flour mixture using your fingertips until well distributed and the mixture feels crumbly/mealy. Using a wooden spoon, stir in walnuts, Gruyere and Parmesan cheeses, and sage, making sure all ingredients are evenly distributed. Place in the refrigerator until needed.
Add pumpkin to a separate small mixing bowl. Using a paper towel, dab the pumpkin puree to soak up as much excess moisture as you can. Whisk in heavy cream and egg until the mixture is evenly combined.
Drizzle the pumpkin mixture over the flour mixture. Stir with a rubber spatula until the flour mixture is thoroughly moistened and a crumbly dough forms; do not overmix.
Using lightly floured hands, gather the dough and gently knead in the mixing bowl until it just comes together. Transfer the dough to a lightly floured surface and form into a 7- to 8-inch-round disc. Cut the disc into 8 equal wedges. Transfer the wedges to the prepared baking sheet.
Generously brush each scone with the brown butter and cream topping mixture and sprinkle with cracked black pepper as desired. Place 1 sage leaf on top of each scone and press gently to adhere.
Bake in the preheated oven until scones are lightly browned, 20 to 23 minutes. Remove from the oven and cool on a wire rack for 5 minutes.