Preheat the oven to 350°F (175°C). Prick the potatoes with a fork and bake directly on the oven rack until tender, about 1 to 1½ hours. Let cool slightly, then peel.
Using a box grater on the small shredding side, grate the peeled potatoes onto a board; discard any very tough outer pieces. Spread the grated potato to cool completely.
When the potatoes are cool, sprinkle ½ teaspoon salt over them, drizzle 2 tablespoons olive oil, then sprinkle about 3/4 of the 2/3 cup flour over the potatoes. Mix with a spatula, then gently knead by hand until a slightly tacky, smooth dough forms; avoid overworking.
Line a baking sheet with parchment and dust with flour. Divide the dough into 4 pieces, roll each into a uniform 1-inch log on a floured surface, cut into 2-inch pieces, and optionally roll each piece over a fork to create grooves. Place gnocchi on the prepared sheet and set aside.
Fill a large pot with water and bring to a boil. Meanwhile heat 2 tablespoons olive oil in a small sauté pan over medium, add the sliced garlic, and cook about 30 seconds until fragrant. Transfer the garlic and oil to a blender.
When the pot is boiling, add the chopped kale and cook 2–3 minutes until stems are slightly tender. Use a slotted spoon to transfer the kale to the blender with the garlic. Add 1½ tablespoons nutritional yeast, 1 tablespoon light miso, 2 teaspoons lemon juice, salt and pepper to taste, and about 1/3 cup of the reserved cooking water. Blend on high until completely smooth, adding more cooking water by tablespoonfuls if needed to reach a saucy consistency. Taste and adjust seasoning.
Bring the kale cooking water back to a boil and cook the gnocchi in batches until they rise to the surface, about 1 minute. Use a slotted spoon to transfer cooked gnocchi directly into the bowl with the kale sauce.
Toss the gnocchi gently with the sauce, adding more reserved cooking water if needed to loosen. Serve hot and finish with optional toppings if desired.