Roast, stirring halfway through, until the gnocchi are plump and some of the tomatoes have burst and caramelized, 15 to 20 minutes total. Remove from the oven and add 12 ounces marinated bite-sized mozzarella balls. Toss to combine and spread in an even layer. Roast until the mozzarella has softened, 2 to 3 minutes more. Garnish with chopped fresh basil leaves.