Heat oven to 400°F. Lightly oil potatoes, prick several times with a fork and sprinkle with salt. Place on lined baking sheet and bake until tender, 1 hour. Meanwhile, place peas in small saucepan with 1 1⁄2 cups water to cover. Slice 4 scallions into 1-inch segments and add to peas. Finely chop remaining scallions and set aside. Bring peas to a simmer over medium-high heat, remove from heat and drain, reserving 1⁄4 cup water. Place pea mixture in blender base with garlic, mint, 1 Tbs. lemon zest, 1⁄2 tsp. salt, and reserved cooking water. Puree until smooth.
Combine rice flour, potato starch and cornstarch in a medium bowl. Let potatoes cool slightly, peel and rice into a large bowl. Stir in 1 cup of pea mixture, eggs, and 2 tsp. salt. Mix in flour until fully incorporated and divide dough into eighths.
Lightly dust a clean work surface with rice flour; line a rimmed baking sheet with parchment and dust with additional rice flour. Working with one section of dough at a time, keep the others tightly wrapped in plastic wrap to avoid drying out. Roll each segment of dough into a log 10 inches long by 1 inch wide. Cut the log into 1"x1" segments and roll each into a 3"x1⁄2" rope. Cut each rope into 1"x1⁄2" gnocchi and place on prepared tray. Rest uncovered at room temperature while preparing the remaining gnocchi, 15 to 20 minutes.
Meanwhile, bring a large pot of salted water to a boil. Place cream in a large skillet and bring to a simmer over medium heat. Reduce heat to low and simmer until reduced by half, 15 to 20 minutes. Stir in reserved scallion and chopped tarragon, and season with lemon juice, salt and white pepper to taste.Add gnocchi to boiling water in two batches and cook 1 1⁄2 to 2 minutes, until gnocchi float on surface. Remove with a slotted spoon and toss with tarragon cream. Garnish with reserved lemon zest and serve.