Preheat grill for medium heat and lightly oil the grate.
Season chicken breasts with garlic powder, salt, and black pepper.
Grill chicken until no longer pink in the center and cooked through, about 6 minutes per side. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C). Cut chicken into strips or bite-sized pieces.
Warm corn and black beans in separate saucepans over medium-low heat, covered, until heated through, about 5 minutes.
Layer cooked rice, corn, black beans, and chicken on 2 serving plates. Top with Monterey Jack cheese, lettuce, salsa, and sour cream.
