In a small bowl, whisk together balsamic vinegar, olive oil, garlic powder, onion powder, salt, and pepper. Reserve 2 tablespoons of the marinade for later use. Place the flank steak in a shallow dish and pour the remaining marinade over it. Cover and refrigerate for at least 30 minutes, or up to 2 hours for more flavor.
Preheat the grill to medium-high heat. Remove the steak from the marinade and discard the used marinade. Grill the steak for about 5-7 minutes on each side, or until it reaches your desired level of doneness. Let the steak rest for 5 minutes before slicing it thinly against the grain.
In a large bowl, combine the mixed salad greens, cherry tomatoes, corn, Gorgonzola cheese, red onion, and avocado. Drizzle the reserved marinade over the salad and toss gently to combine.
Top the salad with the sliced steak and serve immediately.