Heat oven to 425°F; oil a rimmed sheet. Gently mix chicken, ricotta, panko, egg, basil, meatball salt, garlic powder, and pepper. Shape 24 meatballs; bake 16 to 20 minutes until centers reach 165°F.
Meanwhile, heat oil in a saucepan. Cook onion 5 minutes; garlic 30 seconds. Add tomatoes and broth; simmer.
Transfer hot broth mixture and meatballs to a 5-quart slow cooker. Cover; cook on HIGH 2 hours, gently turning once.
Whisk cornstarch with slurry water; fold into sauce. Cover; cook on HIGH 15 minutes until lightly thickened. Turn off cooker; wait until bubbling settles. Fold in cream, Parmesan, and spinach; cover 5 minutes to wilt. Keep meatball centers at least 165°F.
Meanwhile, boil gnocchi in a large pot with gnocchi-water salt for 2 to 3 minutes or package time, until floating and tender. Drain well; gently fold into sauce.
Divide gnocchi, spinach, and sauce among 6 bowls; add 4 meatballs each. Serve promptly before gnocchi absorb more sauce.