Cut 1/2 inch from the top of each head of garlic, exposing the cloves. Soak the heads in a bowl of cold water for 30 minutes to 1 hour.
Preheat a gas grill to high; adjust the heat to low after 15 minutes. (If cooking over charcoal, allow the coals to burn until they are covered with gray ash.) Arrange the garlic heads cut-side up around the perimeter of the grill. Place them to avoid any direct flame, then brush them with the oil. Cook, with the grill covered, for 1 1/2 to 2 hours or until the cloves are very soft. (If using charcoal, add 8 to 10 briquettes after 1 hour to maintain the heat.) Sprinkle with the salt and pepper. To serve, squeeze the garlic from the cloves onto crusty bread and spread.
Rainy-Day Method: Place the prepared garlic heads in a covered baking dish and roast in a 325° F oven 1 1/2 to 2 hours or until soft. Sprinkle with salt and pepper.