Homemade pizza on the grill tastes like wood-fired restaurant pizza. Personalize the recipe by using your favorite vegetable or cheese combinations. Mushrooms, asparagus, squash, or eggplant would also work well for the vegetable toppings.
To prepare dough, dissolve yeast in warm water in a large bowl. Stir in 2 tablespoons oil, and let stand 5 minutes. Lightly spoon all-purpose flour into dry measuring cups; level with a knife. Add 3 1/2 cups flour and salt to yeast mixture; stir until a dough forms. Turn dough out onto a lightly floured surface. Knead until smooth and elastic (about 10 minutes); add enough of remaining flour, 1 tablespoon at a time, to prevent dough from sticking to hands.
Place dough in a large bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place (85°), free from drafts, 1 hour or until doubled in size. (Gently press two fingers into dough. If indentation remains, dough has risen enough.) Punch dough down; cover and let rest 5 minutes.
While dough rises, prepare topping. Place zucchini, bell peppers, and onion in a large bowl; stir in 1 tablespoon oil and black pepper, tossing well. Place on grill rack coated with cooking spray; grill 5 minutes on each side or until tender. Remove from grill; stir in garlic.
Divide dough into 6 equal portions, pressing each portion into a 7-inch round. Coat both sides of dough rounds with cooking spray. Place 3 dough rounds on grill rack. Grill 4 minutes or until puffed and golden. Remove from grill. Turn rounds over; top each round with 1 tablespoon mozzarella, about one-sixth of vegetables, and 1 tablespoon Gruyère cheese. Place topped rounds on grill. Cover and cook 1 1/2 minutes or until bottoms are browned. Repeat procedure with remaining 3 dough rounds, vegetables, and cheeses.