Prepare grill. Coat the grill rack with cooking spray.
Cut an 18 x 12-inch sheet of heavy-duty foil. Place potatoes and garlic in center of foil; sprinkle with 1/8 teaspoon salt and 1/8 teaspoon black pepper. Fold foil over vegetables, tightly sealing edges. Add foil pouch to grill rack. Grill 30 minutes, turning foil pouch occasionally.
Lightly coat onion and bell peppers with cooking spray; sprinkle with 1/8 teaspoon salt and 1/8 teaspoon black pepper. Place onion on grill rack; grill 15 minutes. Turn onion over; add bell peppers and sausage. Grill an additional 15 minutes or until vegetables are tender and sausage is done; turn occasionally. Remove from grill.
Slice bell peppers into strips; coarsely chop onion. Combine bell peppers, onion, and potatoes in a large bowl. Squeeze garlic cloves into a small bowl to extract garlic pulp, and discard skins. Add parsley and oregano, tossing well. Slice grilled sausage links into 1/2-inch-thick slices, and serve with bell pepper mixture.