Combine cumin and garlic-pepper seasoning; press evenly onto Flat Iron Steak. Place steaks on grill over medium, ash-covered coals. Grill, covered, 10 to 14 minutes (over medium heat on preheated gas grill, covered, 12 to 17 minutes) for medium rare (145°F) to medium (160°F) doneness, turning occasionally.
Meanwhile, combine salsa, avocado and cilantro in small bowl.
Carve steaks into slices; season with salt, as desired. Top tortillas with equal amounts of steaks and avocado salsa. Garnish with toppings, as desired.