Heat oven to 375 degrees. Coat 13 x 9 x 2-inch baking dish with nonstick cooking spray.
In large skillet, heat oil over medium-high heat. Add peppers, ground beef and garlic salt; cook, stirring to break up clumps of beef, until the peppers are soft and beef is no longer pink, about 5 minutes. Drain off any excess fat from skillet.