In a large bowl, mix mashed potatoes, pumpkin puree, egg, and salt.
Gradually add flour, kneading gently until a soft dough forms.
Divide the dough into smaller portions and roll each into a ½-inch-thick rope.
Cut into 1-inch pieces and press lightly with a fork to create ridges.
Bring a large pot of salted water to a boil. Cook gnocchi in batches until they float to the surface (2-3 minutes). Remove and drain.
In a skillet, melt butter over medium heat and add sage leaves. Cook until the butter browns and the sage becomes crispy.
Toss the cooked gnocchi in the butter sage sauce. Serve hot, topped with Parmesan if desired.