Combine chicken stock, whole milk and cornstarch in a medium saucepan and stir until cornstarch is dissolved. Cook over medium heat, stirring constantly until bubbling.
Reduce heat to low and continue to cook 5 minutes, until thick. Remove from heat.
Preheat oven to 375.
Toss gluten-free bread (cubed) with olive oil or use an oil sprayer (such as a Misto or Evo) to coat and toss with onion powder, garlic powder, salt and pepper (to taste).
Spread on baking sheet and cook for 20 minutes, stirring halfway through. Optional: Pulse toasted bread in a food processor until desired size. (It should resemble croutons more than bread crumbs.)
Reduce oven heat to 350.
Thinly slice squash and cook in a large pan sprayed with olive oil, covered, on medium heat until softened and liquid is released.
Mix sour cream and cream of chicken soup together in large bowl.
Add carrots and squash (including liquid released during cooking).