Heat Grill to medium-high. Meanwhile, trim ends of zucchini and summer squash and cut rounds about 1/2 -inch thick and 3 inches long. Place in a large bowl and toss with oil, using your hands to evenly coat all sides. Season with garlic salt , pepper and toss again. Place veggies in a single layer on grill and cook until browned with grill marks, 4 to 5 minutes on each side. Transfer to platter and drizzle with balsamic vinegar before serving.