This method of cooking only requires that you be able to enclose the food in some way, charcoal works just as well as gas. With a charcoal grill you simply build the fire on one side of the grill and cook on the other. When using a charcoal grill to cook indirectly I find it best to build the fire like you always would and then use a small metal shovel or similar tool to shift the hot coals to one side.
Of course there are a lot of possibilities when it comes to fire building. With a gas grill you are limited in how you set up the fire. Burners have this annoying habit of either being on or off. When it comes to indirect grilling a burner on low is still too hot so it's off or on, how high depends on your target cooking temperature. However with charcoal you can build all kinds of indirect fires. Coals can be piled in the middle and food placed around the edge, the coals can be around the edge and the food in the middle, the coals can be on the side, well you get the idea.
So what do you do if you have a small gas grill and only one burner. Well on of the tools you need for indirect grilling is a drip pan. This can be a heavy cast iron pan or a disposable aluminum pan. This pan sits under the cooking grate where you plan on doing the cooking. If you have a one burner grill then the drip pan should go in the middle with the food directly over it. The drip pan diverts the rising heat and creates the space you need for indirect grilling. The drip pan also catches all the drippings from the food and helps keep your grill clean.
As I said, you grill indirectly anything that will burn on the surface before it can get cooked through to the middle. This includes cuts of meat over 2 inches in thickness, poultry, roasts, etc. You also use this method for grilling with a rotisserie.