Heat the olive oil in a large pot or Dutch oven over medium-high heat. Add the celery, carrots, onion, 1/2 teaspoon salt and a few grinds of pepper. Cook, stirring occasionally, until the vegetables soften, about 5 minutes. Add the garlic and red pepper flakes; cook 1 minute.
Add 6 cups water, the tomatoes, rosemary, parmesan rind and 1/2 teaspoon salt to the pot. Bring to a simmer, then reduce the heat to medium and cook until the vegetables are very tender, about 15 minutes.Stir in the escarole and cook until wilted, about 3 minutes; season with salt and pepper. Stir in the gnocchi and cook, gently stirring occasionally, until tender, about 3 minutes. Discard the rosemary sprig and parmesan rind. Ladle the soup into bowls; top with the grated parmesan and drizzle with olive oil.