In a small bowl, whisk together balsamic vinegar, olive oil, garlic, oregano, salt, and black pepper.
Place the flank steak in a resealable plastic bag or shallow dish and pour the marinade over it. Seal the bag or cover the dish and marinate in the refrigerator for at least 30 minutes, or up to 4 hours for more flavor.
Preheat the grill to medium-high heat. Remove the steak from the marinade and discard the marinade.
Grill the steak for about 4-5 minutes per side for medium-rare, or until desired doneness. Remove from the grill and let it rest for 5-10 minutes before slicing.
While the steak is resting, prepare the salad. In a large bowl, combine the mixed greens, cherry tomatoes, corn, Gorgonzola cheese, red onion, and avocado. Toss gently to combine.
Slice the grilled steak against the grain into thin strips. Top the salad with the sliced steak and serve immediately.