In a small bowl, whisk together balsamic vinegar, olive oil, minced garlic, salt, and black pepper.
Place the flank steak in a resealable plastic bag and pour the marinade over it. Seal the bag and refrigerate for at least 1 hour 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 5-7 minutes on each side for medium-rare, or until desired doneness.
Remove from the grill and let it rest for 5 minutes before slicing.
While the steak is resting, in a large bowl, combine the mixed salad 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.