Make the Sauce:
Put the tomatoes and their juices in a food processor and pulse 4 or 5 times until the tomatoes are crushed but not completely puréed.
Remove the sausage from its casing and tear the sausage apart with your hands into coarse pieces. Heat 1 Tbs. of the olive oil in a 12-inch skillet over medium-high heat. Add the sausage and cook, further breaking it apart with a wooden spoon, until lightly browned and almost completely cooked through, 3 to 5 minutes. With a slotted spoon, transfer the sausage to a plate.
Add the remaining 2 Tbs. olive oil to the pan and then add the leek, parsley, thyme, marjoram, garlic, and salt. Cook, stirring, until the leek is soft and fragrant, about 3 minutes. Adjust the heat if the garlic or leek shows any sign of burning.
Add the tomatoes and reserved sausage and stir well to combine, scraping up any browned bits on the bottom of the pan. Bring to a vigorous simmer and then reduce the heat to medium-low. Cover the pan with the lid slightly ajar and simmer gently for 45 minutes. If the sauce is bubbling too fast, reduce the heat to low. Remove the lid and if the sauce seems watery, continue to simmer, stirring occasionally, until reduced to a thick sauce consistency. Adjust the seasoning to taste with salt and pepper.
Cook the Gnocchi and Toss with the Sauce:
Bring a large pot of well-salted water to a boil over high heat. To get the gnocchi into the boiling water, fold the parchment ends to form a chute and gently shake the gnocchi out, taking care not to clump them together as you drop them in. Give one gentle stir, wait until the gnocchi all float to the surface of the water, and then cook them for 1 minute. Carefully drain the gnocchi in a colander. Transfer them to the sauce and gently toss to coat. Serve immediately, topped with Parmigiano, if using.
You can make this sauce ahead and refrigerate it, covered, for up to 5 days or freeze it for up to 1 month.