Preheat grill to medium-high heat.
Cut the chicken in half length-wise to create thin slices.
Spray the chicken with cooking spray, then season with garlic powder and black pepper.
Place the chicken onto the grill. Grill for 3-5 minutes per side, until fully cooked.
Top each piece of chicken with a spoonful of marinara sauce and a slice of mozzarella cheese.
Close the grill lid and cook for 1 more minute, until the cheese is melted.
