Peel and finely dice the onion and garlic. Put tomatoes with onion, garlic and herbs in a pot. Bring to the boil, season with salt, pepper and sugar. Let simmer for about 5 minutes.
Cook gnocchi in boiling salted water according to package instructions. Drain and let drain. Season sauce once more. Serve the gnocchi with the sauce. Sprinkle with parmesan and garnish with sage.