Pour a little olive oil into a frying-pan, add the garlic clove and the tomato passata, salt and add the basil, cover and let it cook in a medium-low heat for about 0 minutes. In the meantime, boil plenty of water in a large saucepan for the gnocchi and salt once boiled.
Remove the garlic from the tomato sauce and pour nearly all the sauce into a very deep bowl. Cook the gnocchi in at least 2-3 batches; they need to cook just for a few moments in just-simmering water as boiling water can damage them: take them out with a slotted spoon and drain them carefully as soon as they rise to the surface of the water, and put them into the bowl where the tomato sauce is. Mix them very delicately with a spoon.
Take an ovenproof dish and pour the remaining tomato sauce to cover the bottom, add a little olive oil and then gently add in half the gnocchi. Add a layer of diced mozzarella and sprinkle with parmesan. Then repeat with the rest of the gnocchi, another layer of mozzarella and the rest of the parmesan. Bake the gnocchi in a preheated oven at 250° C in grill setting for 5 minutes.