Cooking with pots on a gas grill is no different than cooking with pots on a gas top stove. The main thing is to make sure to use the proper pots, do not close the top of the gas grill and to keep a close eye on the heat of the gas grill so it does not get too hot. It is an ideal way to cook vegetables or other garnishes alongside your main dish that is already on the grill.
Decide what pots or pans you are going to use on the gas grill. Cast iron is the best type of pot or pan to use on a gas grill. Make sure that there is no plastic evident, that the pan is not made of Teflon and that it has no type of enameling or the pot will get ruined on the gas grill. The exterior could char, become brittle and be a real pain to clean and these can be dangerous to cook with on a gas grill. You may use a more inexpensive pan in case it becomes burned or ends up with too many deposits of grease on the underside of it.
Clean the grill thoroughly in the area you are going to place the pot or pan (i.e. if you are only using one half of the grill grating, and using the other to cook food on the grill). Do this before you put any type of pan on it. If there are drippings from past grilling, such as hamburger or steak drippings, the grease and gunk will build up under the pot and burn the underside or could possibly even start a fire from the creation of oily smoke. Take a grill scraper/brush with paper towels and vigorously scrape off the drippings and charred pieces from the grating.
Turn the gas grill on once you are done cleaning it and ready to start cooking. Let it heat up for about five minutes or so before placing any pots of pans on the grill. Keep the gas grill heated but not too hot. Keep the grill on medium heat. The lower heat may take longer to cook foods, but it will add more tenderness to the meat and vegetables when cooked more slowly.
Keep the pots on the side of the grill when cooking food in them. Before you place any food in the pot or pan, make sure the pan where you place the food is well greased up with Pam, olive oil and seasonings such as salt and pepper if desired. Constantly stir the cooking food with a wooden spoon to make sure the undersides don't burn.
Keep the lid to the gas grill open throughout cooking. If you do place a top of the pan you are using on the gas grill, make sure it has no plastic on it. Keep a close eye on it so you can let steam out and the food doesn't overcook.
Clean the pots and pans thoroughly and be sure to turn off the grill once you are finished cooking. You may designate a particular pan as your gas grill pot or pan so you have one that can always be used for this purpose.