Preheat the oven to 425°F. Line a baking sheet with parchment paper.
Place zucchini, squash, red pepper, green pepper and onion into a bowl. Drizzle with olive oil and season with garlic salt and black pepper.
Preheat grill to medium-high. Oil grates. Place veggies on foil or in a grill pan and saute on medium-high heat for about 15 minutes or just until tender. Set aside to cool slightly.
Unroll the pizza crust onto the baking sheet and form it into an 13 x 8 inch rectangle. Spread half of the pizza sauce over the crust.
Sprinkle half of the shredded cheese, over sauce, season with the red pepper flakes and Italian seasoning.
Arrange the veggies over the sauce then drizzle the remaining sauce over veggies. Sprinkle with reserved mozzarella cheese and Parmesan cheese.
Bake for 15 minutes until golden.
Cut into 8 squares, garnish with parsley and serve while hot.