Place the zucchini, red bell peppers, yellow bell peppers, squash, red onion, mushrooms, and tomatoes in a large bowl.
In a small bowl, combine olive oil, soy sauce, lemon juice, and garlic; mix together. Pour over the vegetables. Cover bowl, and marinate in the refrigerate for 1/2 hour.
Preheat grill for medium heat.
Lightly oil grate. Remove vegetables from marinade, and place on preheated grill. Cook for 12 to 15 minutes, or until tender.