Preheat oven to 400°F. Coat a 13x9-inch baking dish with cooking spray.
Slice zucchini in half lengthwise, and scoop out the centers to create a boat.
In a large skillet over medium heat, cook ground chicken until no longer pink. Season with 1 teaspoon garlic powder; salt and pepper to taste.
Add spaghetti sauce, and simmer until sauce is thickened a bit.
Stir in 2 cups shredded mozzarella and ½ cup Parmesan cheese to chicken mixture; stir until melted.
Spoon mixture into zucchini boats, and sprinkle with remaining mozzarella and Parmesan.
Bake, uncovered, 25 minutes or until zucchini is tender. When done, turn oven to broil; broil 1-2 minutes or until cheese browns.