Heat grill to high. Heat the oil in a large frying pan, then soften the onion and pepper for 5 mins. Stir in the garlic, fry for 1 min, tip in the tomatoes and gnocchi, then bring to a simmer. Bubble for 10-15 mins, stirring occasionally, until the gnocchi is soft and the sauce has thickened. Season, stir through the basil, then transfer to a large ovenproof dish.