Cut 1 garlic clove in half and rub cut sides on inside of a large bowl and both sides of bread slices. Tear bread into 1-inch pieces. Place in bowl and lightly coat with cooking spray. Sprinkle with pepper and salt and toss to coat.
Transfer to a baking sheet and bake until golden, about 15 minutes. Cool completely.
Pulse onion and remaining garlic in a food processor or blender until finely chopped. Add half of the tomatoes and all their juice and puree. Add bread crumbs, parsley, vinegar, oil, pepper, and salt. Process just until blended and pour into a large, nonreactive bowl.
Chop remaining tomatoes. Stir into tomato puree with half of cucumbers and half of bell peppers. Refrigerate at least 2 hours. Ladle chunky gazpacho into bowls and top with remaining cucumber, peppers, and croutons.