Heat gas or charcoal grill. With small metal spoon, scrape underside of mushroom caps to remove dark gills. Spray mushroom caps with cooking spray. Place mushroom caps, underside down, on gas grill over medium-high heat or on charcoal grill 4 to 6 inches from medium-high coals. Cook 8 to 10 minutes or until mushroom caps are tender, turning occasionally. Remove from grill; turn underside up.