Coat grill rack with nonstick cooking spray. Preheat grill to medium (300F to 350F). Coat both sides of steak with nonstick cooking spray. Rub with garlic pepper, and salt to taste.
Grill, covered, 5 to 8 minutes per side (145F for medium-rare) or to desired doneness. Transfer steak to a cutting board; let stand 3 minutes.
Meanwhile, place spinach on 6 plates. Top with bell pepper and onion, and drizzle with vinaigrette. Thinly slice steak against the grain; place on top of salads. If desired, sprinkle evenly with cheese, and serve.