In small bowl, beat eggs, garlic salt and pepper thoroughly with fork or wire whisk. Stir in bell pepper and onions.
In 8-inch skillet, melt butter over medium heat. Pour egg mixture into skillet. As mixture begins to set at bottom and side, gently lift cooked portions with spatula so that thin, uncooked portions can flow to bottom. Avoid constant stirring. Cook 3 to 4 minutes or until eggs are thickened throughout. Gently stir in cheese.
Heat taco shells as directed on package. Place lettuce in shells. Spoon eggs onto lettuce. Top with avocado and salsa.