As the soup will be extremely hot, there should be no need to reheat once the milk or cream is added. However, you can always return the pot to the slow cooker for an extra 10–15 minutes on high ...
Combine onion, carrots, celery, and garlic in the bowl of your slow cooker. Season chicken on ... then pour back into soup. Add cream and gnocchi, cover, and cook 45 minutes more on high.