Preheat an outdoor grill for medium heat and lightly oil the grate.

Season chicken breasts with garlic powder, salt, and black pepper.

Cook chicken on the preheated grill until no longer pink in the center and cooked through, about 6 minutes per side. An instant-read thermometer inserted into centers should read at least 165 degrees F (74 degrees C). Cut chicken into strips or bite-sized pieces.

Heat corn and black beans in two separate saucepans over medium-low heat, covered, until heated through, about 5 minutes.

Layer cooked rice, corn, black beans, and chicken on two serving plates. Top with lettuce, sour cream, Monterey Jack cheese, and salsa.