There are a couple of key things to consider when wanting to cook a pig on a gas grill. Make sure that the grill is large enough for the pig. A typical 30-by-24-inch grill can probably handle up to a 30-pound pig. A 48-by-36-inch may take a 75-pound pig. The key to cooking a pig on a grill is to cook with indirect heat. A pan will help with keep the flames from touching the pig.
Prepare the pig. There may not be anything to this step. If you are going to leave the skin on, then you are done. It does no good to baste a pig that still has the skin on. If you are planning to baste and add flavors, then remove the skin.
Prepare the grill. Get the temperature of the grill to 275 degrees. This is going to be your cooking temperature.
Place the pig on the pan and the pan on the grill. Fill the pan halfway with apple juice. You can use other liquids, but apple juice and vinegar are the most common.
Cook the pig for one hour for every 15 pounds. If you do not have a heavy-duty pig-roasting grill that rotates the pig automatically, then you will have to manually turn the pig yourself. If the pig is under 30 pounds then rotate it once every 30 minutes. If the pig is 30 to 60 pounds, then rotate the pig every 45 minutes. Above 60 pounds can be rotated once every hour.
Punch holes in the skin an hour before the pig is finished. This will allow the excess grease to run out.
Verify the pig is cooked. Using a meat thermometer, check the temperature an hour before it is ready. You want the pig to be at 160 degrees. Be sure to get the thermometer as deep in the center as possible. At 160 degrees, the pig is cooked and ready to be eaten.