Cook ground beef, onion, and green bell pepper in a medium skillet over medium heat until the beef is browned and vegetables are softened. Drain excess fat or liquid.

Stir in garlic powder, mustard, ketchup, and brown sugar until combined.

Reduce heat and simmer for about 30 minutes until the sauce thickens and coats the beef. Season with salt and black pepper.

Serve warm on hamburger buns.